sys: Automated cleanup of cdefs and other formatting

Apply the following automated changes to try to eliminate
no-longer-needed sys/cdefs.h includes as well as now-empty
blank lines in a row.

Remove /^#if.*\n#endif.*\n#include\s+<sys/cdefs.h>.*\n/
Remove /\n+#include\s+<sys/cdefs.h>.*\n+#if.*\n#endif.*\n+/
Remove /\n+#if.*\n#endif.*\n+/
Remove /^#if.*\n#endif.*\n/
Remove /\n+#include\s+<sys/cdefs.h>\n#include\s+<sys/types.h>/
Remove /\n+#include\s+<sys/cdefs.h>\n#include\s+<sys/param.h>/
Remove /\n+#include\s+<sys/cdefs.h>\n#include\s+<sys/capsicum.h>/

Sponsored by: Netflix

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

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

net: clean up empty lines in .c and .h files

Use host byte order when comparing mss values.

This fixes tcp-setmss action on little endian machines.

PR: 225536
Submitted by: John Zielinski

Use the new VNET_DEFINE_STATIC macro when we are defining static VNET
variables.

Reviewed by: bz
Sponsored by: DARPA, AFRL
Differential Revision: https://reviews.freebsd.org/D16147

Fix IPv6 extension header parsing. The length field doesn't include the
first 8 octets.

Obtained from: Yandex LLC
MFC after: 3 days

Add ipfw_pmod kernel module.

The module is designed for modification of a packets of any protocols.
For now it implements only TCP MSS modification. It adds the external
action handler for "tcp-setmss" action.

A rule with tcp-setmss action does additional check for protocol and
TCP flags. If SYN flag is present, it parses TCP options and modifies
MSS option if its value is greater than configured value in the rule.
Then it adjustes TCP checksum if needed. After handling the search
continues with the next rule.

Obtained from: Yandex LLC
MFC after: 2 weeks
Relnotes: yes
Sponsored by: Yandex LLC
No objection from: #network
Differential Revision: https://reviews.freebsd.org/D10150