bridging: Add support for TOUCH_MIXMONITOR_OPTIONS

bridging: Add support for TOUCH_MIXMONITOR_OPTIONS

This channel variable can be used to override the default automixmon
options (which are 'b' or 'bB(<n>)'). If both TOUCH_MIXMONITOR_OPTIONS
and TOUCH_MIXMONITOR_BEEP are set, TOUCH_MIXMONITOR_BEEP is ignored with
a warning.
It is the responsibility of the user to configure valid options in
TOUCH_MIXMONITOR_OPTIONS.

Fixes: #1981
UserNote: The TOUCH_MIXMONITOR_OPTIONS channel variable can now be used
to configure the options used by the mixmonitor application when it is
started via automixmon. If both TOUCH_MIXMONITOR_OPTIONS and
TOUCH_MIXMONITOR_BEEP are set, TOUCH_MIXMONITOR_BEEP is ignored with
a warning.