notify the decoder instead of polling 100hz

When the decoder process is faster than the player process, all
decodedd buffers are full at some point in time.  The decoder has to
wait for buffers to become free (finished playing).  It used to do
this by polling the buffer status 100 times a second.

This generates a lot of unnecessary CPU wakeups.  This patch adds a
way for the player process to notify the decoder process that it may
continue its work.

We could use pthread_cond for that, unfortunately inter-process
mutexes/conds are not supported by some kernels (Linux), so we cannot
use this light-weight method until mpd moves to using threads instead
of processes.  The other method would be semaphores, which
historically are global resources with a unique name; this historic
API is cumbersome, and I wanted to avoid it.

I came up with a quite naive solution for now: I create an anonymous
pipe with pipe(), and the decoder process reads on that pipe.  Until
the player process sends data on it as a signal, the decoder process
blocks.

This can be optimized in a number of ways:

- if the decoder process is still working (instead of waiting for
buffers), we could save the write() system call, since there is
nobody waiting for the notification.
[ew: I tried this using a counter in shared memory, didn't help]

- the pipe buffer will be full at some point, when the decoder thread
is too slow.  For this reason, the writer side of the pipe is
non-blocking, and mpd can ignore the resulting EWOULDBLOCK.

- since we have shared memory, we could check whether somebody is
actually waiting without a context switch, and we could just not
write the notification byte.
[ew: tried same method/result as first point above]

- if there is already a notification in the pipe, we could also not
write another one.
[ew: tried same method/result as first/third points above]

- the decoder will only consume 64 bytes at a time.  If the pipe
buffer is full, this will result in a lot of read() invocations.
This does not hurt badly, but on a heavily loaded system, this might
add a little bit more load.  The preceding optimizations however
are able eliminate the this.

- finally, we should use another method for inter process
notifications - maybe kill() or just make mpd use threads, finally.

In spite of all these possibilities to optimize this code further,
this pipe notification trick is faster than the 100 Hz poll.  On my
machine, it reduced the number of wakeups to less than 30%.

git-svn-id: https://svn.musicpd.org/mpd/trunk@7215 09075e82-0dd4-0310-85a5-a0d7c8717e4f

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