MFC 359047,359054: Deprecate procfs-based process debugging.

359047:
Mark procfs-based process debugging as deprecated for FreeBSD 13.

Attempting to use ioctls on /proc/<pid>/mem to control a process will
trigger warnings on the console. The <sys/pioctl.h> include file will
also now emit a compile-time warning when used from userland.

359054:
Fix the workaround to ignore the #warning for GCC.

clang and gcc use different warning flags for #warning preprocessor
directives.

For both 12 and 11, adjust the GCC warning flags to only be added in
4.7 and later since 4.2.1 does not support -Wno-cpp. For 11, add the
needed warning suppression to procctl's build. procctl was removed in
12.0.

Copy head@r302406 to stable/11 as part of the 11.0-RELEASE cycle.
Prune svn:mergeinfo from the new branch, as nothing has been merged
here.

Additional commits post-branch will follow.

Approved by: re (implicit)
Sponsored by: The FreeBSD Foundation

The last big commit: let usr.sbin/ use WARNS=6 by default.

Perform a major cleanup of the usr.sbin Makefiles.
These are not perfectly in agreement with each other style-wise, but they
are orders of orders of magnitude more consistent style-wise than before.

Set WARNS=2 on programs that compile cleanly with it; add $FreeBSD$
where necessary.

Submitted by: Mike Barcroft <mike@q9media.com>

- Backout botched attempt to introduce MANSECT feature.
- MAN[1-9] -> MAN.

Set the default manual section for usr.sbin/ to 8.

$Id$ -> $FreeBSD$

explicitly set MAN8 since only MAN1 is defaulted.

delete -static, add $Id$

First checkin of the procctl program.