This special case was useless.  Fixes the dreaded "Unrecognized URI"
error message when a file does not exist.

"override" implies "virtual".

Found by "make distcheck".

If an async opertion is in progress, nfs_close_async() will make
libnfs crash because the RPC callback will dereference an object that
was freed by nfs_close_async().

Close the newly allocated file handle passed to the callback.

All locks are currenly held from only a single thread (the IOThread)
and thus we don't need the Mutex.

Not used anymore because NfsManager now uses boost::intrusive::set
instead of std::map.

Reduce overhead for storing the key twice, and more overhead while
looking up the connection to remove it after a failure.

This allows eliminating the indirection code from the NfsConnection
destructor.

It's just a useless wrapper for class NfsManager.

Reduce Remove() overhead because we don't have to walk the list to
find an iterator by reference.

The new tag is supported by all decoders that use the tag name table,
and the ID3v2 tag "TSOA" maps to it.

Assume the configured filesystem character set is also used by the NFS
server.

Swap the order of adding the URI and the slash, because appending is
cheaper than inserting.

The Release Track Id uniquely identifies a recording on a release - that
is, even if a recording appears twice on a release (meaning that the
combination of recording and release id are not enough to figure out
which one it is), the release track id will allow differentiating the two.

The tag names are taken from
https://musicbrainz.org/doc/MusicBrainz_Picard/Tags/Mapping

There's always the "PCM" decoder plugin, which was never checked by
configure.ac.

MPD can easily be used as a database provider for the proxy database
plugin.  In that case, it needs only one "null" output, and no real
output plugin.

Allow building a smaller MPD binary for people who don't need DSD.

The name "cue" was listed twice in "mpd --version".