Fix 'security.bsd.see_jail_proc' by using cr_bsd_visible()

As implemented, this security policy would only prevent seeing processes
in sub-jails, but would not prevent sending signals to, changing
priority of or debugging processes in these, enabling attacks where
unprivileged users could tamper with random processes in sub-jails in
particular circumstances (conflated UIDs) despite the policy being
enforced.

PR: 272092
Reviewed by: mhorne
MFC after: 2 weeks
Sponsored by: Kumacom SAS
Differential Revision: https://reviews.freebsd.org/D40628

sys: Remove $FreeBSD$: one-line .c pattern

Remove /^[\s*]*__FBSDID\("\$FreeBSD\$"\);?\s*\n/

Move most of the contents of opt_compat.h to opt_global.h.

opt_compat.h is mentioned in nearly 180 files. In-progress network
driver compabibility improvements may add over 100 more so this is
closer to "just about everywhere" than "only some files" per the
guidance in sys/conf/options.

Keep COMPAT_LINUX32 in opt_compat.h as it is confined to a subset of
sys/compat/linux/*.c. A fake _COMPAT_LINUX option ensure opt_compat.h
is created on all architectures.

Move COMPAT_LINUXKPI to opt_dontuse.h as it is only used to control the
set of compiled files.

Reviewed by: kib, cem, jhb, jtl
Sponsored by: DARPA, AFRL
Differential Revision: https://reviews.freebsd.org/D14941

Remove BSD and USL copyright and update license block in in_prot.c, as the
code in this file was written by Robert N. M. Waston.

Move cr_can* prototypes from sys/systm.h to sys/proc.h

Reported by: rwatson
Reviewed by: rwatson
Approved by: sjg (mentor)
Differential Revision: https://reviews.freebsd.org/D7345

Prepare for network stack as a module

- Move cr_canseeinpcb to sys/netinet/in_prot.c in order to separate the
INET and INET6-specific code from the rest of the prot code (It is only
used by the network stack, so it makes sense for it to live with the
other network stack code.)
- Move cr_canseeinpcb prototype from sys/systm.h to netinet/in_systm.h
- Rename cr_seeotheruids to cr_canseeotheruids and cr_seeothergids to
cr_canseeothergids, make them non-static, and add prototypes (so they
can be seen/called by in_prot.c functions.)
- Remove sw_csum variable from ip6_forward in ip6_forward.c, as it is an
unused variable.

Reviewed by: gnn, jtl
Approved by: sjg (mentor)
Sponsored by: Juniper Networks, Inc.
Differential Revision: https://reviews.freebsd.org/D2901