Multiplexor bugs

[utzoo!decvax!watmath!arwhite]

This bug causes a gp to be treated as a cp and a crash may result if you do
a NPGRP or CSIG in error on a group rather than channel (or as I found it;
by passing -1 as the channel number!)
-------mx1.c---------
417c417
< 			if (cp==NULL || cp->c_flags&ISGRP) {
---
> 			if (cp==NULL) {
434c434
< 		if (cp==NULL || cp->c_flags&ISGRP) {
---
> 		if (cp==NULL) {
If you have an old multiplexor inode sitting around from before a real crash,
then the return of the null group[d] value will be carefully checked in mxopen
which will return - unfortunately not having set u.u_error - thus the channel
is actually returned as opened, but no tables have been set up!
SYMPTOMS:  System crashes.  On the way back up there is an attempt to preserve
editor files - which invokes mail to notify you it has done so.  Mail trys
to open /dev/mail to send you notice.  comsat is not yet up; /dev/mail is from
before the crash; and the system crashes in the close after successfully openingit
-------mx2.c---------
45c45
< 	if (d >= NGROUPS || groups[d] == NULL) {
---
> 	if (d >= NGROUPS) {