bsdgrep: add -z/--null-data support

-z treats input and output data as sequences of lines terminated by a
zero byte instead of a newline. This brings it more in line with GNU grep
and brings us closer to passing the current tests with BSD grep.

Submitted by: Kyle Evans <kevans91 at ksu.edu>
Reviewed by: cem
Relnotes: Yes
Differential Revision: https://reviews.freebsd.org/D10101