core: introduce separate VOCTRLs for screensaver stop/resume

This is slightly better because VOCTRL_RESUME/VOCTRL_PAUSE are usually
needed by VOs to know whether video is actually being played (for
whatever reason), and they wouldn't be passed to the backend's VOCTRL
handler, like vo_x11_control().

Also try to make sure that these flags (both pause state and screensaver
state) are set consistently in some corner cases. For example, it seems
enabling video in the middle of playing a file while the player is
paused would not set the paused flag.

If codec initialization fails, destroy the VO instead of keeping it
around to make sure the state is consistent.

Framestepping is implemented by unpausing the player for the duration of
a frame. Remove the special handling of VOCTRL_PAUSE/RESUME in these
cases. It was most likely needed because these VOCTRLs used to be
important for screen redrawing (blatant guess), which is now handled
completely differently. The only potentially bad side-effect is that the
screensaver will be disabled/reenabled for the duration of one frame.
x11: enable screensaver when paused, rename/change --stop-xscreensaver

Use the recently introduced screensaver VOCTRLs to control the
screensaver in the X11 backend. This means the behavior when paused
changes: the old code always kept the screensaver disabled, but now the
screensaver is reenabled on pausing.

Rename the --stop-xscreensaver option to --stop-screensaver and make it
more generic. Now it affects all backends that respond to the
screensaver VOCTRLs.
w32: enable screensaver when paused

This is quite similar to the previous commit.

Untested. I'm not sure if this is how it's supposed to work. At least
--no-stop-screensaver should work in any case.
command: use more standard time format for clock property

%k is not very standard. The manpage notes them as conforming to
"Olson's timezone package", and it's not standard C89, C99 or POSIX.
mingw doesn't provide it, and even some of the smaller Linux libcs
don't have support.

Use %H instead. This gives slightly different results, but I think
this is ok. Difference in behavior between these summarized:

  %k: "single digits are preceded by a blank"
  %H: "range 00 to 23"