misc: add a synchronization helper

This is almost like rendezvous(), except it allows async wakeup, and
does not require global state. It will be used by a later commit.

struct mp_waiter is intended to be allocated on the stack, and uses an
initializer including PTHREAD_MUTEX_INITIALIZER. This is the first case
in mpv that it uses PTHREAD_MUTEX_INITIALIZER for stack-allocated
mutexes. It seems POSIX still does not allow this formally, but since
POSIX is worth less than used toilet paper, I don't really care. Modern
OSes use futexes, which means you can make _every_ memory location a
lock, and this code tries to make use of it, without using OS specific
code.

The name of the source file is rather generic, because I intend to dump
further small helpers there (or maybe move mp_rendezvous() to it).

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