sys/crypto: import md5c.c C rewrite

The reimplementation is a bit cleaner than the original code,
although it is also slightly slower. This shouldn't matter too
much as we will have asm code for the major platforms.

The code is unrolled when built in the kernel or user space, but
not when in libsa, as to reduce the code size.

Differential Revision: https://reviews.freebsd.org/D45670
Reviewed by: jrtc27, imp
Approved by: markj (mentor)
MFC after: 1 month