Avoid some duplicated code in main_notify.c.

The function wait_main_task() is racy: if the function
wakeup_via_cond() sees the mutex is locked just before
wait_main_task() executes pthread_cond_wait(), the main thread blocks
forever.

Work around this issue by adding a "pending" flag just like in my
notify.c code.  A standards-compliant solution should be implemented
later.

git-svn-id: https://svn.musicpd.org/mpd/trunk@7365 09075e82-0dd4-0310-85a5-a0d7c8717e4f

pthread_cond_signal is a no-op if nothing is waiting on it

git-svn-id: https://svn.musicpd.org/mpd/trunk@7351 09075e82-0dd4-0310-85a5-a0d7c8717e4f

signal is all we need since we only have one waiter and
likely faster

git-svn-id: https://svn.musicpd.org/mpd/trunk@7349 09075e82-0dd4-0310-85a5-a0d7c8717e4f

Oops, forgot to unlock a mutex

git-svn-id: https://svn.musicpd.org/mpd/trunk@7348 09075e82-0dd4-0310-85a5-a0d7c8717e4f

The select() in the main event loop blocks now (saving us many
unnecessary wakeups).  This interacted badly with the threads
that were trying to wakeup the main task via
pthread_cond_signal() since the main task was not blocked
on a condition variable, but on select().

So now if we detect a need to wakeup the player, we write
to a pipe which select() is watching instead of blindly
calling pthread_cond_signal().

git-svn-id: https://svn.musicpd.org/mpd/trunk@7347 09075e82-0dd4-0310-85a5-a0d7c8717e4f