Move code from the FAAD decoder plugin.

This loop is completely unnecessary.  We just need to find the first
ADTS frame and feed it into NeAACDecInit().

Eliminate duplicate cleanup code.

Previously, MPD tried to slurp the whole song file, count the number
of frames and calculate the song duration from that.  That however is
extremely expensive for remote files, and will delay playback for a
long time.  Workaround: check only the first 128 frames and try to
extrapolate from here.  Fixes Mantis ticket 0004035.

Eliminate more duplicate code.

Don't bother to check for ADIF just because the stream is not
seekable.

Merge some duplicate code.

Eliminate some overhead when the caller doesn't need the buffer.

Checking the integer is faster, easier and more reliable.

.. instead of declaring a new constant.

Replaces the loop in sndfile_vio_read(), eliminating duplicate and
fragile code.

.. instead of InputStream::LockRead(). The former is cancellable.

Implement a "bulk" edit mode that postpones both UpdateQueuedSong()
and OnModified().  This way, the playlist version gets incremented
only once.  More importantly: when adding multiple songs to a queue
that consists of only one song, the first song that got added will
always be played next.  By postponing this choice, all newly added
songs get a chance to become the next song.  Fixes the second (and
last) part of Mantis ticket 0004005.

Don't restore the current song after shufflung when MPD is stopped
(but still remembers the current song internally).  Fixes the first
part of Mantis ticket 0004005.

The XDG cache directory is part of the "base directories" spec like
$XDG_CONFIG_HOME, not "user directories".