[9fans] small smtp fix

[erik quanstrom]

with imap users, it's easy for addresses to be unqualified.
this is partially imap4d's fault, but client misconfiguration
can also cause this.  this patch will send notification of
a bad address right away instead of trying this sort of
invalid address for days:

smtp.c:188,198 - /n/dump/2009/0219/sys/src/upas/smtp/smtp.c:188,193
                usage();
        addr = *argv++; argc--;
        farend = addr;
-       if((rv = strrchr(addr, '!')) && rv[1] == '['){
-               syslog(0, "smtp.fail", "%s to %s failed: illegal address",
-                       deliverytype(), addr);
-               exits(Giveup);
-       }

i added this hack to nupas on sources.

- erik