Add sysconf(8) and libbsdconf(3)HEADmain

Complete the native configuration trinity: sysctl(8) for live kernel
state, sysrc(8) for rc.conf(5), and sysconf(8) for the remaining base
configuration -- loader.conf(5), sysctl.conf(5), and the make.conf(5)
family -- atop libbsdconf(3).

libbsdconf resurrects figpar as a unified reader/writer.  Callbacks own
semantics; statements may span multiple lines via backslash continuation;
non-seekable input is spooled; writes are atomic (mkstemp, fsync, rename)
with mode/owner preservation.  Format descriptors name each target, its
files, and quoting rules without private parsers.  Multi-file targets
follow boot sourcing order; loader chases loader_conf_files as the boot
loader does.

sysconf(8) is the operator-facing tool: name / name=value on a required
target, sysrc-style list edits, make append and list-strike where they
belong, jail/altroot, and a capsicum sandbox for read-only use.

Sysctl writes validate against the running kernel first -- unknown and
read-only OIDs, CTLFLAG_TUN (pointing at the loader target), and CTLTYPE
range checks -- so a typo or overflow does not land in sysctl.conf.

Make and src treat WITH_/WITHOUT_ as presence knobs (as bsd.mkopt.mk /
src.conf(5) do) and warn on the WITH_*=no form that does not disable the
option, so a bad assignment is caught before an /usr/src build surfaces
it.

The rc target passes through to sysrc(8).

Defaults querying (-d/-D/-A) mirrors sysrc for dumps and descriptions on
targets that have a defaults file; named reads already see defaults, and
-A only widens dump scope.

Manuals are split pkg(8)-style (bsdconf/put/format; sysconf plus
per-target pages).  ATF coverage exercises the frontend.

Co-authored-by:	Faraz Vahedi <kfv@FreeBSD.org>
Reviewed by:	fuz, kfv
Differential Revision:	https://reviews.freebsd.org/D58066

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