grep: properly switch EOL indicator with -z

-z is supposed to use only the NUL byte as EOL, but we were
inadvertently using both newline and NUL due to REG_NEWLINE in cflags.

The odds of anyone relying on this bsdgrep-specific bug are quite low,
so let's just fix it. At least one port in the wild has been reported
to expect the intended behavior.

Reported by: Hill Ma <maahiuzeon@gmail.com>
Triaged by: the self-proclaimed peanut gallery on Discord

(cherry picked from commit e898a3af97f97f74c0fb22032bbd163f7cc92a05)