Implement SA_NOCLDWAIT.

The implementation is done (unlike what i've originally been
contemplating) by reparenting kids of processes that have the
appropriate bit set to PID 1, and let PID 1 handle the zombie. This
is far less problematical than what would seem to be ``doing it
right'', for a number of reasons.

Of our currently shipping PID-1-intended programs, 50 % fail the above
assumption. ;-) (Read this: sysinstall doesn't do it right. This is
no problem as long as no program called by sysinstall actually uses
SA_NOCLDWAIT.)

ToDo: . clarify the correct SA_* flag inheritance, compared

		  to other systems,
		. decide whether the compat cruft (osigvec(9)) should
		  deal with new system additions or not,
		. merge OpenBSD's SA_SIGINFO implementation. ;)

Reviewed by: bde