Use libmd hashing function implementations instead of embedding our own

This splits the implementation responsibilities, and reduces embedded
code copies, which was one of the driving points with this project to
start with, so it's nice to give a good example.

src/libbsd.map

@@ -35,6 +35,8 @@ LIBBSD_0.0 {
     strunvis;
     strunvisx;
 
+    /* The following functions are provided via the system libc or libmd
+     * implementations. */
     MD5Init;
     MD5Update;
     MD5Pad;