Remove $FreeBSD$: one-line sh pattern

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

Add HAS_TESTS to all Makefiles that are currently using the
`SUBDIR.${MK_TESTS}+= tests` idiom.

This is a follow up to r321912.

Convert traditional ${MK_TESTS} conditional idiom for including test
directories to SUBDIR.${MK_TESTS} idiom

This is being done to pave the way for future work (and homogenity) in
^/projects/make-check-sandbox .

No functional change intended.

MFC after: 1 weeks

Use src.opts.mk in preference to bsd.own.mk except where we need stuff
from the latter.

Migrate tools/regression/{usr.bin/lastcomm,usr.sbin}/ to the new tests layout.

This change was originally going to only migrate the usr.sbin tests but, as
it turns out, the usr.sbin/sa/ tests require files from usr.bin/lastcomm/
so it's better to just also migrate the latter at the same time. The other
usr.bin tests will be moved separately.

To make these tests work within the test suite, some of them have required
changes to prevent modifying the source directory and instead just rely on
the current directory for file manipulation.

The etcupdate utility is a tool for managing updates to files that are
not updated as part of `make installworld' such as files in /etc. It
manages updates by doing a three-way merge of changes made to these files
against the local versions. It is also designed to minimize the amount
of user intervention with the goal of simplifying upgrades for clusters
of machines.

The primary difference from mergemaster is that etcupdate requires less
manual work. The primary difference from etcmerge is that etcupdate
updates files in-place similar to mergemaster rather than building a
separate /etc tree.

Requested by: obrien, kib, theraven, joeld (among others)