Move code shared between libpq and backend from backend/libpq/ to common/.

When building libpq, ip.c and md5.c were symlinked or copied from
src/backend/libpq into src/interfaces/libpq, but now that we have a
directory specifically for routines that are shared between the server and
client binaries, src/common/, move them there.

Some routines in ip.c were only used in the backend. Keep those in
src/backend/libpq, but rename to ifaddr.c to avoid confusion with the file
that's now in common.

Fix the comment in src/common/Makefile to reflect how libpq actually links
those files.

There are two more files that libpq symlinks directly from src/backend:
encnames.c and wchar.c. I don't feel compelled to move those right now,
though.

Patch by Michael Paquier, with some changes by me.

Discussion: <69938195-9c76-8523-0af8-eb718ea5b36e@iki.fi>

src/include/common/ip.h

@@ -0,0 +1,37 @@
+/*-------------------------------------------------------------------------
+ *
+ * ip.h
+ *	  Definitions for IPv6-aware network access.
+ *
+ * These definitions are used by both frontend and backend code.
+ *
+ * Copyright (c) 2003-2016, PostgreSQL Global Development Group
+ *
+ * src/include/common/ip.h
+ *
+ *-------------------------------------------------------------------------
+ */
+#ifndef IP_H
+#define IP_H
+
+#include "getaddrinfo.h"		/* pgrminclude ignore */
+#include "libpq/pqcomm.h"		/* pgrminclude ignore */
+
+
+#ifdef	HAVE_UNIX_SOCKETS
+#define IS_AF_UNIX(fam) ((fam) == AF_UNIX)
+#else
+#define IS_AF_UNIX(fam) (0)
+#endif
+
+extern int pg_getaddrinfo_all(const char *hostname, const char *servname,
+				   const struct addrinfo * hintp,
+				   struct addrinfo ** result);
+extern void pg_freeaddrinfo_all(int hint_ai_family, struct addrinfo * ai);
+
+extern int pg_getnameinfo_all(const struct sockaddr_storage * addr, int salen,
+				   char *node, int nodelen,
+				   char *service, int servicelen,
+				   int flags);
+
+#endif   /* IP_H */

src/include/common/md5.h

@@ -0,0 +1,30 @@
+/*-------------------------------------------------------------------------
+ *
+ * md5.h
+ *	  Interface to libpq/md5.c
+ *
+ * These definitions are needed by both frontend and backend code to work
+ * with MD5-encrypted passwords.
+ *
+ * Portions Copyright (c) 1996-2016, PostgreSQL Global Development Group
+ * Portions Copyright (c) 1994, Regents of the University of California
+ *
+ * src/include/common/md5.h
+ *
+ *-------------------------------------------------------------------------
+ */
+#ifndef PG_MD5_H
+#define PG_MD5_H
+
+#define MD5_PASSWD_LEN	35
+
+#define isMD5(passwd)	(strncmp(passwd, "md5", 3) == 0 && \
+						 strlen(passwd) == MD5_PASSWD_LEN)
+
+
+extern bool pg_md5_hash(const void *buff, size_t len, char *hexsum);
+extern bool pg_md5_binary(const void *buff, size_t len, void *outbuf);
+extern bool pg_md5_encrypt(const char *passwd, const char *salt,
+			   size_t salt_len, char *buf);
+
+#endif