Copy stable/9 to releng/9.3 as part of the 9.3-RELEASE cycle.

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

MFC r239130: Minor style(9) improvement

Copy head to stable/9 as part of 9.0-RELEASE release cycle.

Approved by: re (implicit)

Fix a few more SYSCTL_PROC() that were missing a CTLFLAG type specifier.

Where if not in examples we should follow style(9)?

According to module(9), when command is unrecognized, the module should
return EOPNOTSUPP.

Unbreak by syncing with changes in <sys/sysctl.h> macros.

These patches implement dynamic sysctls. It's possible now to add
and remove sysctl oids at will during runtime - they don't rely on
linker sets. Also, the node oids can be referenced by more than
one kernel user, which means that it's possible to create partially
overlapping trees.

Add sysctl contexts to help programmers manage multiple dynamic
oids in convenient way.

Please see the manpages for detailed discussion, and example module
for typical use.

This work is based on ideas and code snippets coming from many
people, among them: Arun Sharma, Jonathan Lemon, Doug Rabson,
Brian Feldman, Kelly Yancey, Poul-Henning Kamp and others. I'd like
to specially thank Brian Feldman for detailed review and style
fixes.

PR: kern/16928
Reviewed by: dfr, green, phk