Remove $FreeBSD$: one-line .c pattern

Remove /^[\s*]*__FBSDID\("\$FreeBSD\$"\);?\s*\n/

sys.kern.pdeathsig.signal_delivered_ptrace: fix startup.

Inform D that C executed procctl(PROC_PDEATHSIG_CTL). Otherwise D
might allow B to exit before C is set up to receive a signal on the
parent exit. In this case, C waits forever for the signal and test
hangs.

PR: 237657
Reported and tested by: lwhsu
Sponsored by: The FreeBSD Foundation
MFC after: 1 week

sys.kern.pdeathsig.signal_delivered_ptrace: fix debugger detach

ptrace(PT_DETACH) requires stopped debuggee, otherwise it fails. When
the call fails, the C process is left as debuggee of the process D,
and might be killed too early if process D exits occurs fast enough.

Since pipes are not closed in the forked children, this resulted in
the test hanging, since no write occured from C to wake A.

PR: 237657
Reported and tested by: lwhsu
Sponsored by: The FreeBSD Foundation
MFC after: 1 week

Remove redundant pipe from pdeathsig.c test.

A pipe was was left over from a development version of pdeathsig.c and
is not needed.

Process C waits for a signal that'll be generated when process B
exists. Process B waits for process D to send it a byte via pipe_db
before it exits. Process D sends the byte after it has started
ptrace()ing process C. The point of the test is to show that process C
receives the signal because process B exited, even though C has been
reparented to process D. The pipe pipe_cd isn't doing anything useful
(though in an earlier version of the patch it did). Clean that up by
removing the useless pipe.

Submitted by: Thomas Munro
MFC after: 6 days
Differential revision: https://reviews.freebsd.org/D15214

Rename PROC_PDEATHSIG_SET -> PROC_PDEATHSIG_CTL and PROC_PDEATHSIG_GET
-> PROC_PDEATHSIG_STATUS for consistency with other procctl(2)
operations names.

Requested by: emaste
Sponsored by: The FreeBSD Foundation
MFC after: 13 days

Add PROC_PDEATHSIG_SET to procctl interface.

Allow processes to request the delivery of a signal upon death of
their parent process. Supposed consumer of the feature is PostgreSQL.

Submitted by: Thomas Munro
Reviewed by: jilles, mjg
MFC after: 2 weeks
Differential revision: https://reviews.freebsd.org/D15106