Refactor MD5 implementations according to new cryptohash infrastructure

This commit heavily reorganizes the MD5 implementations that exist in
the tree in various aspects.

First, MD5 is added to the list of options available in cryptohash.c and
cryptohash_openssl.c.  This means that if building with OpenSSL, EVP is
used for MD5 instead of the fallback implementation that Postgres had
for ages.  With the recent refactoring work for cryptohash functions,
this change is straight-forward.  If not building with OpenSSL, a
fallback implementation internal to src/common/ is used.

Second, this reduces the number of MD5 implementations present in the
tree from two to one, by moving the KAME implementation from pgcrypto to
src/common/, and by removing the implementation that existed in
src/common/.  KAME was already structured with an init/update/final set
of routines by pgcrypto (see original pgcrypto/md5.h) for compatibility
with OpenSSL, so moving it to src/common/ has proved to be a
straight-forward move, requiring no actual manipulation of the internals
of each routine.  Some benchmarking has not shown any performance gap
between both implementations.

Similarly to the fallback implementation used for SHA2, the fallback
implementation of MD5 is moved to src/common/md5.c with an internal
header called md5_int.h for the init, update and final routines.  This
gets then consumed by cryptohash.c.

The original routines used for MD5-hashed passwords are moved to a
separate file called md5_common.c, also in src/common/, aimed at being
shared between all MD5 implementations as utility routines to keep
compatibility with any code relying on them.

Like the SHA2 changes, this commit had its round of tests on both Linux
and Windows, across all versions of OpenSSL supported on HEAD, with and
even without OpenSSL.

Author: Michael Paquier
Reviewed-by: Daniel Gustafsson
Discussion: https://postgr.es/m/20201106073434.GA4961@paquier.xyz

src/common/md5_int.h

@@ -0,0 +1,85 @@
+/*-------------------------------------------------------------------------
+ *
+ * md5_int.h
+ *	  Internal headers for fallback implementation of MD5
+ *
+ * Portions Copyright (c) 1996-2020, PostgreSQL Global Development Group
+ * Portions Copyright (c) 1994, Regents of the University of California
+ *
+ * IDENTIFICATION
+ *		  src/common/md5_int.h
+ *
+ *-------------------------------------------------------------------------
+ */
+
+/*	   $KAME: md5.h,v 1.3 2000/02/22 14:01:18 itojun Exp $	   */
+
+/*
+ * Copyright (C) 1995, 1996, 1997, and 1998 WIDE Project.
+ * All rights reserved.
+ *
+ * Redistribution and use in source and binary forms, with or without
+ * modification, are permitted provided that the following conditions
+ * are met:
+ * 1. Redistributions of source code must retain the above copyright
+ *	  notice, this list of conditions and the following disclaimer.
+ * 2. Redistributions in binary form must reproduce the above copyright
+ *	  notice, this list of conditions and the following disclaimer in the
+ *	  documentation and/or other materials provided with the distribution.
+ * 3. Neither the name of the project nor the names of its contributors
+ *	  may be used to endorse or promote products derived from this software
+ *	  without specific prior written permission.
+ *
+ * THIS SOFTWARE IS PROVIDED BY THE PROJECT AND CONTRIBUTORS ``AS IS'' AND
+ * ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
+ * IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
+ * ARE DISCLAIMED.  IN NO EVENT SHALL THE PROJECT OR CONTRIBUTORS BE LIABLE
+ * FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
+ * DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
+ * OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
+ * HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
+ * LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
+ * OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
+ * SUCH DAMAGE.
+ */
+
+#ifndef PG_MD5_INT_H
+#define PG_MD5_INT_H
+
+#include "common/md5.h"
+
+#define MD5_BUFLEN 64
+
+/* Context data for MD5 */
+typedef struct
+{
+	union
+	{
+		uint32		md5_state32[4];
+		uint8		md5_state8[16];
+	}			md5_st;
+
+#define md5_sta     md5_st.md5_state32[0]
+#define md5_stb     md5_st.md5_state32[1]
+#define md5_stc     md5_st.md5_state32[2]
+#define md5_std     md5_st.md5_state32[3]
+#define md5_st8     md5_st.md5_state8
+
+	union
+	{
+		uint64		md5_count64;
+		uint8		md5_count8[8];
+	}			md5_count;
+#define md5_n   md5_count.md5_count64
+#define md5_n8  md5_count.md5_count8
+
+	unsigned int md5_i;
+	uint8		md5_buf[MD5_BUFLEN];
+} pg_md5_ctx;
+
+/* Interface routines for MD5 */
+extern void pg_md5_init(pg_md5_ctx *ctx);
+extern void pg_md5_update(pg_md5_ctx *ctx, const uint8 *data, size_t len);
+extern void pg_md5_final(pg_md5_ctx *ctx, uint8 *dest);
+
+#endif							/* PG_MD5_INT_H */