bsdgrep: bump version number and add Kyle Evans copyright

The following changes have been made over the last couple of months:

Features:

• With bsdgrep -r, the working directory is implied if no directory is specified
• bsdgrep will now behave as bsdgrep -r does when it's named rgrep
• bsdgrep now understands -z/--null-data to use \0 as EOL
• GNU regex compatibility is now indicated with a "GNU compatible" in the version string

Fixes:

• --mmap no longer hangs when coming across an EOF without an accompanying EOL
• -o/--color matching generally improved, now produces earliest / longest matches
• Context output now more closely aligns with GNU grep
• Zero-length matches no longer exhibit broken behavior
• Every output line now honors -b/-H/-n flags

Tests have been added for previous regressions as well as other
previously untested behaviors.

Various other fixes have been commited, and refactoring for further /
later improvements has taken place.

(The original submission changed the version string to 2.5.2, but I
decided to use 2.6.0 to reflect the addition of new features.)

Submitted by: Kyle Evans <kevans91@ksu.edu>
Differential Revision: https://reviews.freebsd.org/D10982