Fixes assertion failure in destructor by not assigning
{src,dest}_format when an error occurs.

Fixes truncated lines in m3u and cue files (regression by commit
67958f7f).

The file systemd/mpd.service.in is being distributed implicitly, but
systemd/mpd.socket is not and needs to be added to EXTRA_DIST.

Fixes deadlock bug in the "mms" plugin.

Export it from the protected base class.  This fixes a build failure
on Mac OS X.

Fixes buffer overflow bug.

MPD handles all strings in UTF-8 internally.  Those decoders which
read Latin-1 tags are supposed to implement the conversion, instead of
passing Latin-1 to TagBuilder::AddItem().  FixTagString() is simply
the wrong place to do that, and hard-coding Latin-1 is kind of
arbitrary.

Share the NFS connection with the NFS input plugin.

Auto-stop the IO thread in all error handlers.

Read all directory entries into memory and close the struct nfsdir
before returning the StorageDirectoryReader instance.  This is what
libnfs does, anyway.