Implement protocol version check in dovecot-keylookupd
A change in the major dict protocol version (see #4) caused dovecot-keylookupd to silently fail with recent Dovecot clients, due to an error in parsing the L ("lookup") command now that it has additional fields.
Dovecot-keylookupd should parse the H ("hello") line and reject clients with versions it does not support.