bsdgrep(1): Slooowly peel away the chunky onion

(or peel off the band-aid, whatever floats your boat)

This addresses two separate issues:

1.) Nothing within bsdgrep actually knew whether it cared about line numbers

or not.

2.) The file layer knew nothing about the context in which it was being

called.

#1 is only important when we're *not* processing line-by-line. #2 is
debatably a good idea; the parsing context is only handy because that's
where we store current offset information and, as of this commit, whether or
not it needs to be line-aware.