it is definately wrong to initialize the pgid at open() time because the
uid/euid used for checking in csignal() is controlled by whoever used
TIOCSPGRP previously.

for non-tty TIOCSPGRP/F_SETOWN/FIOSETOWN pgid setting calls, store uid
and euid as well, then deliver them using new csignal() interface
which ensures that pgid setting process is permitted to signal

for non-tty TIOCSPGRP/F_SETOWN/FIOSETOWN pgid setting calls, store uid
and euid as well, then deliver them using new csignal() interface
which ensures that pgid setting process is permitted to signal the
pgid process(es). Thanks to newsham@aloha.net for extensive help and
discussion.

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partial sync with netbsd 960418, more to come

From NetBSD: 960217 merge

initial import of NetBSD tree