player: increase seek accuracy when refreshing display on filter change

When the player is paused, and video filters are changed, an exact seek
is executed to refresh the display. Increase the exactness of the seek
in this case; this reuses the code used for frame backstepping.

It might help in cases where seeking is very imprecise, such as with
transport streams.
manpage: fix a minor aspect of the discnav command

"menu" is in fact understood by stream_bluray.c, so just drop that
sentence.
threads: use mpv time for mpthread_cond_timedwait wrapper

Use the time as returned by mp_time_us() for mpthread_cond_timedwait(),
instead of calculating the struct timespec value based on a timeout.
This (probably) makes it easier to wait for a specific deadline.
options: unify code for setting string and "raw" options

The code paths for setting options by string and by direct "raw" value
were too different, which resulted in some weird code. Make the code
paths closer to each other.

Also, use this to remove the weirdness in the mpv_set_option()
implementation.
client API: add mpv_load_config_file()

This is probably a good idea, because it would make it easier for
software embedding mpv to configure the mpv parts, without requiring the
host program to provide explicit mechanisms for this (other than calling
mpv_load_config_file()).
vdpau: don't fallback to software decoding on preemption

This was requested by someone. Not sure if it's a good idea; it seems
more can go wrong than right.