Heres an outline of things that'd need to be done to support multiple
character transfers.  Note that nothing need be broken until the last
step when the new functions are enabled.  Everything else can be done 
while retaining the original interface.

* new linesw function for multiple char transfer: l_mrint
	/sys/include/conf.h	add l_mrint to linesw structure
	/sys/kern/tty_conf.c	add NULL entries for l_mrint initialization

* check the new l_mrint in com.c: if not NULL, use it rather than l_rint
	/sys/dev/isa/com.c
	/* other places? */

* add l_mrint interface stubs for slip and ppp
	/sys/net/if_sl.c	slmultinput(int *p, int size, struct tty *tp)
	/sys/net/if_ppp.c	pppmultinput(int *p, int size, struct tty *tp)

* modify the slip and ppp stub functions to work ;)
	in if_sl.c, this would essentially entail wrapping the code from
	slinput() in a while loop.  this would be a first step.
	i assume if_ppp.c would be the same way.

* "turn on" the minput routines by setting the l_mrint pointer to the
	proper function.