Remove $FreeBSD$: one-line sh pattern

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

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.

proccontrol(1) man page.

Reviewed by: 0mp
Sponsored by: The FreeBSD Foundation
Differential revision: https://reviews.freebsd.org/D19308

Use MAN=<empty> rather than MK_MAN=no to not install a manpage.

MFC after: 1 week

Provide proccontrol(1), an utility to control processes behaviour, related
to procctl(2).

Sponsored by: The FreeBSD Foundation
MFC after: 1 week