simplify mu-plugin management

the Drop-in mechanism does not need to specify a file-name, in which case it copies all files (excluding readme.txt and similars). Slightly less stuff to write in this case: plugins which declare themselfs to be mu-plugins will be handled automaticaly, the ones which do not have to be listed twice.

More-privacy-option plugin actually ships with other files (translations), so it should not be considered a mu-plugin, but installed network wide as a regular plugin.