bsdgrep: add BSD_GREP_FASTMATCH knob for built-in fastmatch

Bugs have been found in the fastmatch implementation as used in bsdgrep.
Some have been fixed (r316495) while fixes for others are in review
(D10098).

In comparison with the fastmatch implementation, Kyle Evans found that:

• regex(3)'s performance with literal expressions offers a speed improvement over fastmatch

• regex(3)'s performance, both with simple BREs and EREs, seems to be comparable

The regex implementation was imported in r226035, and the commit message
reports:

This is a temporary solution until the whole regex library is
not replaced so that BSD grep development can continue and the
backported code gets some review and testing. This change only
improves scalability slightly, there is no big performance boost
yet but several minor bugs have been found and fixed.

Introduce a WITH_/WITHOUT_BSD_GREP_FASTMATCH knob to support testing
of both approaches.

PR: 175314, 194823
Submitted by: Kyle Evans <kevans91 at ksu.edu>
Reviewed by: bdrewery (in part)
Differential Revision: https://reviews.freebsd.org/D10282