Rename "register_ethereal_tap()" to "register_tap_listener_cmd_arg()" as

Rename "register_ethereal_tap()" to "register_tap_listener_cmd_arg()" as
it's used to register a callback for a tap listener invoked if the
specified command line argument is specified to the "-z" flag.

Move it, along with routines to:

	look up a "-z" argument in the table constructed by
	"register_tap_listener_cmd_arg()" and either save the full
	argument to "-z" and the corresponding listener if it's found or
	return a failure indication if it isn't;

	list the available tap listeners;

	call the "init" routines for the tap listeners saved in the
	table above;

and have Ethereal and Tethereal use those routines.

svn path=/trunk/; revision=13993