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

Remove /^/[*/]\s*\$FreeBSD\$.*\n/

dhclient: fix time parsing for leases expiring after 2038

Convert lease parsing to timegm to calculate timestamp. For reference, when
writing the lease, we use gmtime to convert the timestamp to struct tm.

Reviewed By: markj, vangyzen
MFC after: 2 weeks
Sponsored by: Dell EMC Isilon
Differential Revision: https://reviews.freebsd.org/D40760

dhclient(8): Fix a typo in a source code comment

- s/explicitely/explicitly/

MFC after: 3 days

dhclient: raise WARNS to 4

Mostly const-correctness fixes. There were also some variable-shadowing,
unused variable, and a couple of sockaddr type-correctness changes. I also had
trouble with cast-align warnings. I was able to prove that one of them was a
false positive. But ultimately I had to disable the warning program-wide to
deal with the others.

Reviewed by: cem
MFC after: 3 weeks
Sponsored by: Spectra Logic Corp
Differential Revision: https://reviews.freebsd.org/D14460

Fix -Wunused-but-set-warning with `ret`

While here, resolve Coverity warnings by demonstrating that vfprintf's
return value is being explicitly ignored.

MFC after: 1 week
Reported by: gcc 6.3.0
Tested with: clang, gcc 4.2.1, gcc 6.3.0
Sponsored by: Dell EMC Isilon

Migrate tools/regression/sbin/ to the new tests layout.

Pretty much all that this change does is shuffles the code around and hooks
it into the regular build. The code of the old tests has not changed.