dispatch: redo locking, and allow reentrant processing

A deadlock bug was reported with the following test program:

	mpv_handle *mpv = mpv_create();
	mpv_set_option_string(mpv, "ytdl", "yes");
	mpv_initialize(mpv);
	mpv_terminate_destroy(mpv);

The cause of this is loading the ytdl.lua script, which triggers a
certain code path that calls mp_dispatch_queue_process() recursively. It
does so to wait until the script is loaded, and we want to keep that.

Reentrancy was not supported by mp_dispatch, which leads to the
deadlock. Rewrite the locking so that it does. We mainly get rid of the
"exclusive_lock" mutex. Instead we use the existing lock/condition
variable to wait until we can grab a logical lock.

Note that the lock_frame business can be replaced with a simple counter.
Instead of checking the lock_frame address, it'd simply increment and
store the counter when entering mp_dispatch_queue_process(), and then
compare the counter to decide whether or not to wait. But I think the
additional error checking done by the lock_frame list is valuable.

Fixes #3489.

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