as with the stop command, this will cause the player and decoder
to suspend and not wake up hundreds of times a second to poll
a variable for wakeup.  This will reduce power consumption
on some CPUs while mpd is paused and not playing.

tests:

pause && unpause => OK

pause && stop && play => OK

pause && exit && restart w/statefile && unpause => OK

pause && block sound device && \
unpause => failed to open sound device \
=> still paused and suspended => unblock sound device &&
unpause => OK (playing)

In all cases, the player process releases the audio device
when paused before going into the suspended state.

git-svn-id: https://svn.musicpd.org/mpd/trunk@6822 09075e82-0dd4-0310-85a5-a0d7c8717e4f