Improve some code around cryptohash functions

This adjusts some code related to recent changes for cryptohash
functions:
- Add a variable in md5.h to track down the size of a computed result,
moved from pgcrypto.  Note that pg_md5_hash() assumed a result of this
size already.
- Call explicit_bzero() on the hashed data when freeing the context for
fallback implementations.  For MD5, particularly, it would be annoying
to leave some non-zeroed data around.
- Clean up some code related to recent changes of uuid-ossp.  .gitignore
still included md5.c and a comment was incorrect.

Discussion: https://postgr.es/m/X9HXKTgrvJvYO7Oh@paquier.xyz

contrib/pgcrypto/internal.c

@@ -41,10 +41,6 @@
 #include "common/cryptohash.h"
 #include "common/md5.h"
 
-#ifndef MD5_DIGEST_LENGTH
-#define MD5_DIGEST_LENGTH 16
-#endif
-
 #ifndef SHA1_DIGEST_LENGTH
 #ifdef SHA1_RESULTLEN
 #define SHA1_DIGEST_LENGTH SHA1_RESULTLEN

contrib/uuid-ossp/.gitignore

@@ -1,4 +1,3 @@
-/md5.c
 /sha1.c
 # Generated subdirectories
 /log/

contrib/uuid-ossp/uuid-ossp.c

@@ -41,8 +41,8 @@
 #undef uuid_hash
 
 /*
- * Some BSD variants offer md5 and sha1 implementations but Linux does not,
- * so we use a copy of the ones from pgcrypto.  Not needed with OSSP, though.
+ * Some BSD variants offer sha1 implementation but Linux does not, so we use
+ * a copy from pgcrypto.  Not needed with OSSP, though.
  */
 #ifndef HAVE_UUID_OSSP
 #include "sha1.h"