signal 0

[Doug Gwyn <gwyn>]

I couldn't find the original message about using signal 0 to determine
whether a child process has terminated without doing a wait for it.
Since then, friends have researched the issue, with the following result:

	signal 0 NOT SUPPORTED on 6th Edition UNIX, 7th Edition UNIX,
	nor 4.1BSD

	signal 0 SUPPORTED on UNIX System III, UNIX System V, Research
	Version 8, and 4.2BSD