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).
More fgrep tests have been added to more extensively test this one-off
MFC after: 2 weeks?