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/interfaces/libpq/fe-connect.c

@@ -72,7 +72,7 @@ static int ldapServiceLookup(const char *purl, PQconninfoOption *options,
 				  PQExpBuffer errorMessage);
 #endif
 
-#include "libpq/ip.h"
+#include "common/ip.h"
 #include "mb/pg_wchar.h"
 
 #ifndef FD_CLOEXEC