bsdgrep: add a primitive literal matcher

fgrep/grep -F will error out at runtime if compiled with a regex(3)
that does not define REG_NOSPEC or REG_LITERAL. glibc is one such regex(3)
implementation, and as it turns out they don't support literal matching at
all.

Provide a primitive literal matcher for use with glibc and other
implementations that don't support literal matching so that we don't
completely lose fgrep/grep -F if compiled against libgnuregex on stable/10,
stable/11, or other systems that we don't necessarily support.

This is a wholly unoptimized implementation with no plans to optimize it as
of now. This is due to both its use-case being primarily on unsupported
systems in the near-distant future and that it's reinventing the wheel that
we already have available as a feature of regex(3).

Reviewed by: cem, emaste, ngie
Approved by: emaste (mentor)
MFC after: 2 weeks
Differential Revision: https://reviews.freebsd.org/D12056