Use implicit <md5.h> from overlay instead explicit <bsd/md5.h>
This was assuming an installed <bsd/md5.h> on the system, due to the
build system not including -Iinclude/ anymore.
Regression introduced in commit 901ed630fc64fca828e031bc8fa6780db9f05db1.
Replace current md5 code with one from MirBSD and OpenBSD
This adds the following public functions:
MD5Transform, MD5End, MD5File, MD5FileChunk, MD5Data
