When the decoder receives SIGCONT during waitNotify(), the kernel
restarts the read() system call.  This lets the decoder process block
indefinitely, while the player process waits for it to react.  This
should probably be solved with a proper signal handler which aborts
the read() system call, but for now, we just write to the pipe to make
it wake up.

git-svn-id: https://svn.musicpd.org/mpd/trunk@7216 09075e82-0dd4-0310-85a5-a0d7c8717e4f