bsdgrep: remove output separators between overlapping segments

Make bsdgrep more sensitive to context overlaps. If it's printing
context that either overlaps or is immediately adjacent to another bit
of context, don't print a separator.

• Non-overlapping segments no longer have two separators between them

• Overlapping segments no longer have separators between them with overlapping sections repeated

Submitted by: Kyle Evans <kevans91 at ksu.edu>
Reviewed by: cem
Differential Revision: https://reviews.freebsd.org/D10105