Add a flag to suppress the "default_print()" call made in various

Add a flag to suppress the "default_print()" call made in various
link-layer print routines if no other print routine claimed the packet.
Test whether that flag is set rather than testing whether neither of -x
or -q were specified, and have -x, -q, *and* -X set that flag, so that
-X suppresses it just as -x does.  That way you don't get those pckets
dumped twice if -X was specified.