Replace NetBSD grep with OpenBSD grep

The NetBSD version is seriously broken--it is unable to match lines
against an empty pattern--and appears to be unmaintained.

The new version is the latest OpenBSD grep, with a number of MINIX-
specific changes marked as such, and an additional number of
(signed/unsigned, const) fixes to pass compilation.

Since this is not NetBSD code, move back the entire thing into
minix/usr.bin.

Change-Id: Icd40794a2d0cff9e7fb452db7d28c16dbd25e51d
libsys: resolve clang warnings

Change-Id: Ic954ba8667b4d039172b8e0d2ec57674a479b8aa