Add generic message authentication interface

Add generic interface for generating and verifying MACs (message
authentication codes). Replace multiple HMAC implementation with
a generic one.

lib/Doc

@@ -2,6 +2,7 @@ H Library functions
 S ip.c
 S lists.c
 S checksum.c bitops.c patmatch.c printf.c xmalloc.c tbf.c
+S mac.c
 D resource.sgml
 S resource.c
 S mempool.c