libinput-seat: allow setting libinput log priority in weston

libinput-seat: allow setting libinput log priority in weston

Look for WESTON_LIBINPUT_LOG_PRIORITY environment variable.  If
it exists then use it to set the libinput log priority.
Otherwise, don't set the priority and get whatever libinput's
default priority is.

Setting WESTON_LIBINPUT_LOG_PRIORITY=0 allows us to log which
input devices are detected at Weston startup and makes it a
little more consistent with Weston's original evdev input setup
log messages... and useful for debugging and testing.

Signed-off-by: U. Artie Eoff <ullysses.a.eoff@intel.com>