sys: Remove $FreeBSD$: two-line .h pattern

Remove /^\s*\*\n \*\s+\$FreeBSD\$$\n/

sys: further adoption of SPDX licensing ID tags.

Mainly focus on files that use BSD 3-Clause license.

The Software Package Data Exchange (SPDX) group provides a specification
to make it easier for automated tools to detect and summarize well known
opensource licenses. We are gradually adopting the specification, noting
that the tags are considered only advisory and do not, in any way,
superceed or replace the license texts.

Special thanks to Wind River for providing access to "The Duke of
Highlander" tool: an older (2014) run over FreeBSD tree was useful as a
starting point.

Renumber copyright clause 4

Renumber cluase 4 to 3, per what everybody else did when BSD granted
them permission to remove clause 3. My insistance on keeping the same
numbering for legal reasons is too pedantic, so give up on that point.

Submitted by: Jan Schaumann <jschauma@stevens.edu>
Pull Request: https://github.com/freebsd/freebsd/pull/96

Make net.inet.ip.sourceroute, net.inet.ip.accept_sourceroute, and
net.inet.ip.process_options vnet-aware. Revert changes in r271545.

Suggested by: bz

In ip_stripoptions():
- Remove unused argument and incorrect comment.
- Fixup ip_len after stripping.

A new jail(8) with a configuration file, to replace the work currently done
by /etc/rc.d/jail.

Add function ip_checkrouteralert(), which will be used
by IGMPv3 to check for the IPv4 Router Alert [RFC2113]
option in a pulled-up IP mbuf chain.

- Import the HEAD csup code which is the basis for the cvsmode work.

Normalize style a bit: reduce pseudo-randomness of comment layout and
white space. Remove 'register'.

Move MAX_IPOPTLEN and struct ipoption back into ip_var.h as
userland programs depend on it.

Pointed out by: le
Sponsored by: TCP/IP Optimization Fundraise 2005

Consolidate all IP Options handling functions into ip_options.[ch] and
include ip_options.h into all files making use of IP Options functions.

From ip_input.c rev 1.306:
ip_dooptions(struct mbuf *m, int pass)
save_rte(m, option, dst)
ip_srcroute(m0)
ip_stripoptions(m, mopt)

From ip_output.c rev 1.249:
ip_insertoptions(m, opt, phlen)
ip_optcopy(ip, jp)
ip_pcbopts(struct inpcb *inp, int optname, struct mbuf *m)

No functional changes in this commit.

Discussed with: rwatson
Sponsored by: TCP/IP Optimization Fundraise 2005