Use `pausing_keep_force` to stop MPlayer starting playback while paused

`pausing_keep_force` was introduced in 2008 (r27665), and while still
marked as experimental, should be stable for getting and setting simple
properties.

`pausing_keep` causes MPlayer to advance one frame every time a command
is issued.

Fixes issue #11.
Pre-compile regular expression, streamline match checking
Ensure MPlayer never shows a video output

Be overly paranoid with MPlayer arguments.