Fix some issues reported by lintian

See merge request !2

Changes-By: lintian-brush
Fixes: lintian: missing-built-using-field-for-golang-package
See-also: https://lintian.debian.org/tags/missing-built-using-field-for-golang-package.html

Changes-By: lintian-brush
Fixes: lintian: global-files-wildcard-not-first-paragraph-in-dep5-copyright
See-also: https://lintian.debian.org/tags/global-files-wildcard-not-first-paragraph-in-dep5-copyright.html

Changes-By: lintian-brush
Fixes: lintian: uses-debhelper-compat-file
See-also: https://lintian.debian.org/tags/uses-debhelper-compat-file.html

Changes-By: lintian-brush
Fixes: lintian: package-uses-old-debhelper-compat-version
See-also: https://lintian.debian.org/tags/package-uses-old-debhelper-compat-version.html

Changes-By: lintian-brush
Fixes: lintian: package-lacks-versioned-build-depends-on-debhelper
See-also: https://lintian.debian.org/tags/package-lacks-versioned-build-depends-on-debhelper.html

Changes-By: lintian-brush
Fixes: lintian: build-depends-on-obsolete-package
See-also: https://lintian.debian.org/tags/build-depends-on-obsolete-package.html

Changes-By: lintian-brush
Fixes: lintian: trailing-whitespace
See-also: https://lintian.debian.org/tags/trailing-whitespace.html

By default Dovecot old_stats will create the socket owned by root (non
overridable from the configuration). Thus switch to a dedicated socket
and provide an example configuration.

Buster compatibility

See merge request !1

Buster ships with Dovecot 2.3, thus switch to "old_stats".

See also https://doc.dovecot.org/configuration_manual/stats/old_statistics/

See also ai3/prod#15

add Dockerfile

Update wiki links.

The dovecot stats module referenced here has been deprecated.

Support scraping of "global" scope

PR #5 (itself based on #4) introduced support to scrape the "global" scope,
however the format of these stats differs slightly from that of the
others.

As a result of this, combined with the silent return when `len(values)
!= len(columns)+1`, these metrics were being incorrectly exported as
mislabeled metrics with values of 0.

The changes in this PR accomodate for the format of the "global" scope
and now properly scrape those as well.

It will now also display an error when column counts mismatch, reducing
the likelihood of silent errors in the future.

Global metrics will now show up as:

```
dovecot_global_auth_cache_hits 0
dovecot_global_auth_cache_misses 0
dovecot_global_auth_db_tempfails 3
dovecot_global_auth_failures 6
dovecot_global_auth_master_successes 0
dovecot_global_auth_successes 228
dovecot_global_clock_time 0
dovecot_global_disk_input 0
dovecot_global_disk_output 0
dovecot_global_invol_cs 0
dovecot_global_last_update 1.516197189175826e+09
dovecot_global_mail_cache_hits 0
dovecot_global_mail_lookup_attr 0
dovecot_global_mail_lookup_path 0
dovecot_global_mail_read_bytes 0
dovecot_global_mail_read_count 0
dovecot_global_maj_faults 0
dovecot_global_min_faults 0
dovecot_global_num_cmds 0
dovecot_global_num_connected_sessions 234
dovecot_global_num_logins 234
dovecot_global_read_bytes 0
dovecot_global_read_count 0
dovecot_global_reset_timestamp 1.516190181e+09
dovecot_global_sys_cpu 0
dovecot_global_user_cpu 0
dovecot_global_vol_cs 0
dovecot_global_write_bytes 0
dovecot_global_write_count 0
dovecot_up{scope="global"} 1
```

...with the other scopes being unaffected.

At Kumina, we use an ancient version of Dovecot on some of our systems
that doesn't support this scope yet.

The default set of scopes to be scraped was changed in #5.

We use this script at Kumina to generate binaries that we can deploy to
a wide variety of Linux systems. Let's keep this in the tree, like we do
for some of our other exporters.

Discussed in: #5

Switch to kingpin and make scopes configurable

This allows a user to select which sets of dovecot stats he or she is
interested in.

The default has been deliberately changed mainly because it makes the
description/format more obvious, but also because gathering global stats
in addition to user stats generally makes sense.

Original credit goes to @jap
(https://github.com/kumina/dovecot_exporter/pull/4)

Prometheus is standardizing on kingping for flag parsing, as discussed
in https://github.com/prometheus/prometheus/issues/2455. Many exporters
are following this decision so most of the Prometheus ecosystem uses
kingpin.

As a consequence of this change, flags now require double dashes.
`-web.listen-address` becomes `--web.listen-address` for example.

Usage before:

```
Usage of ./dovecot_exporter:
  -dovecot.socket-path string
    	Path under which to expose metrics. (default "/var/run/dovecot/stats")
  -web.listen-address string
    	Address to listen on for web interface and telemetry. (default ":9166")
  -web.telemetry-path string
    	Path under which to expose metrics. (default "/metrics")
```

And after:

```
usage: dovecot_exporter [<flags>]

Prometheus metrics exporter for Dovecot

Flags:
  --help                        Show context-sensitive help (also try --help-long and --help-man).
  --web.listen-address=":9166"  Address to listen on for web interface and telemetry.
  --web.telemetry-path="/metrics"
                                Path under which to expose metrics.
  --dovecot.socket-path="/var/run/dovecot/stats"
                                Path under which to expose metrics.
```

The existing parser can crash cause out of bounds array access in two
different cases:

- The first line being empty.
- Successive lines using a different number of columns.

This should fix #2.