Fix handling of pts(4) device names in comsat(8). Also catch fork() errors.

Fix handling of pts(4) device names in comsat(8). Also catch fork() errors.

Pseudo-terminals allocated with posix_openpt(2) will have more slashes
in their path names than comsat(8) allows, so allow slashes when the
character device name starts with "pts/".

This patch is loosely based on NetBSD's changes, revision 1.33. Because
it also included the changes to fork(), I imported them here as well.
Maybe we could import even more fixes from the other BSD's?

Original commit message from the NetBSD folks:

	PR/30170: Markus W Kilbinger: src/libexec/comsat complains
	about: '/' in "/dev/pts/1"

Reported by:	Robert Huff <roberthuff rcn com>