Remove residual blank line at start of Makefile

This is a residual of the $FreeBSD$ removal.

MFC After: 3 days (though I'll just run the command on the branches)
Sponsored by: Netflix

Remove $FreeBSD$: one-line sh pattern

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

build: provide a default WARNS for all in-tree builds

The current default is provided in various Makefile.inc in some top-level
directories and covers a good portion of the tree, but doesn't cover p

build: provide a default WARNS for all in-tree builds

The current default is provided in various Makefile.inc in some top-level
directories and covers a good portion of the tree, but doesn't cover parts
of the build a little deeper (e.g. libcasper).

Provide a default in src.sys.mk and set WARNS to it in bsd.sys.mk if that
variable is defined. This lets us relatively cleanly provide a default WARNS
no matter where you're building in the src tree without breaking things
outside of the tree.

Crunchgen has been updated as a bootstrap tool to work on this change
because it needs r365605 at a minimum to succeed. The cleanup necessary to
successfully walk over this change on WITHOUT_CLEAN builds has been added.

There is a supplemental project to this to list all of the warnings that are
encountered when the environment has WARNS=6 NO_WERROR=yes:
https://warns.kevans.dev -- this project will hopefully eventually go away
in favor of CI doing a much better job than it.

Reviewed by: emaste, brooks, ngie (all earlier version)
Reviewed by: emaste, arichardson (depend-cleanup.sh change)
Differential Revision: https://reviews.freebsd.org/D26455

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Add libregex, connect it to the build

libregex is a regex(3) implementation intended to feature GNU extensions and
any other non-POSIX compliant extensions that are deemed worthy.

These extensions

Add libregex, connect it to the build

libregex is a regex(3) implementation intended to feature GNU extensions and
any other non-POSIX compliant extensions that are deemed worthy.

These extensions are separated out into a separate library for the sake of
not cluttering up libc further with them as well as not deteriorating the
speed (or lack thereof) of the libc implementation.

libregex is implemented as a build of the libc implementation with LIBREGEX
defined to distinguish this from a libc build. The reasons for
implementation like this are two-fold:

1.) Maintenance- This reduces the overhead induced by adding yet another
regex implementation to base.

2.) Ease of use- Flipping on GNU extensions will be as simple as linking
against libregex, and POSIX-compliant compilations can be guaranteed with a
REG_POSIX cflag that should be ignored by libc/regex and disables extensions
in libregex. It is also easier to keep REG_POSIX sane and POSIX pure when
implemented in this fashion.

Tests are added for future functionality, but left disconnected for the time
being while other testing is done.

Reviewed by: cem (previous version)
Differential Revision: https://reviews.freebsd.org/D12934

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