Remove $FreeBSD$: one-line sh pattern

Remove /^\s*#[#!]?\s*\$FreeBSD\$.*$\n/

posixshm: Add a -j option to posixshmcontrol ls, to specify a jail
PR: 257556
Reported by: grembo@

Do a sweep and remove most WARNS=6 settings

Repeating the default WARNS here makes it slightly more difficult to
experiment with default WARNS changes, e.g. if we did something absolutely
bananas and introduced a WARNS=7 and wanted to try lifting the default to
that.

Drop most of them; there is one in the blake2 kernel module, but I suspect
it should be dropped -- the default WARNS in the rest of the build doesn't
currently apply to kernel modules, and I haven't put too much thought into
whether it makes sense to make it so.

Add posixshmcontrol(1) page.

Reviewed by: emaste
With input by: danfe
Sponsored by: The FreeBSD Foundation (kib)
MFC after: 1 week
Differential revision: https://reviews.freebsd.org/D20430

Add posixshmcontrol(1), an utility to manipulate posix shared memory segments.

See usage for the command line structure. Man page will come shortly.

Reviewed by: jilles, tmunro
Sponsored by: The FreeBSD Foundation
MFC after: 1 week
Differential revision: https://reviews.freebsd.org/D20258