vo_opengl: change gamma suboption to take a value

The previous version of the gamma suboption was pretty useless. It could
be used to disable delayed gamma enabling, which is a mechanism to avoid
having to adjust gamma in the shader by default.

Repurpose the suboption and allow setting an exact gamma value with it.
You can already override gamma with the --gamma option as well as the
gamma input property, but these use a weird curve to create the
impression of a linear perceived brightness change when changing the
value. This suboption now allows setting an exact gamma value.
command: format volume property as integer for OSD

The value range is 0-100, so fractional values don't make much sense.
But the underlying data type is probably float to avoid getting "stuck"
when doing small volume increments. So step this around and pretend it's
an integer just on display.
command: use the step size for "add volume" commands

The step argument for "add volume <step>" was ignored until now. Fix it.

There is one problem: by defualt, "add volume" should use the value set
with --volstep. This value is 3 by default. Since the default volue for
the step argument is always 1 (and we don't really want to make the
generic code more complicated by introducing custom step sizes), we
simply multiply the step argument with --volstep to keep it compatible.

The --volstep option should probably be just removed in the future.
DOCS: kill the rest of the old mplayer tech documentation

Not really useful anymore, other than confusing everyone.
ao: document some functions
client API: wait for remaining asynchronous requests before terminating

Sending an asynchronous request and then calling mpv_destroy() would
crash the player when trying to send the reply to the removed client.
Fix this by waiting until all remaining replies have been sent.
client API: rename MPV_EVENT_PLAYBACK_START, add MPV_EVENT_SEEK

Rename MPV_EVENT_PLAYBACK_START to MPV_EVENT_FILE_LOADED.

Add MPV_EVENT_SEEK and MPV_EVENT_PLAYBACK_RESTART.
manpage: lua: update from previous commit

I forgot about this.
manpage: lua: fix typo
audio: add enum name for speaker id
client APIs: fix some typos
client API: add two properties, 'time-start' and 'seekable'
manpage: document properties added in previous commit
config: fix --config-dir logic for global config files

Global config files should be loaded only when --no-config is missing
_and_ --config-dir is not set.
config: don't load global config files with --config-dir

This sidestepped the usual logic by hardcoding the path.
lua: add option to disable auto-loading of lua scripts
msg: add --msgtime option to add timestamps to each output message

Will be helpful to track down strange wait times and such issues, as
well when you have develop something timing related. (Then you may print
timestamps in your debug output, and the --msgtime timestamps will help
giving context.)
timer: add utility function to get relative time
lua: fix use of renamed function

Apparently this was overlooked when get_timer was renamed to get_time.
client API: fix timeout handling

(Again.)

Fixed Lua timers as well.