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/Makefile

@@ -39,10 +39,10 @@ OBJS += chklocale.o inet_net_ntop.o noblock.o pgstrcasecmp.o pqsignal.o \
 	thread.o
 # libpgport C files that are needed if identified by configure
 OBJS += $(filter crypt.o getaddrinfo.o getpeereid.o inet_aton.o open.o system.o snprintf.o strerror.o strlcpy.o win32error.o win32setlocale.o, $(LIBOBJS))
-# backend/libpq
-OBJS += ip.o md5.o
-# utils/mb
+# src/backend/utils/mb
 OBJS += encnames.o wchar.o
+# src/common
+OBJS += ip.o md5.o
 
 ifeq ($(with_openssl),yes)
 OBJS += fe-secure-openssl.o
@@ -96,7 +96,7 @@ backend_src = $(top_srcdir)/src/backend
 chklocale.c crypt.c getaddrinfo.c getpeereid.c inet_aton.c inet_net_ntop.c noblock.c open.c system.c pgsleep.c pgstrcasecmp.c pqsignal.c snprintf.c strerror.c strlcpy.c thread.c win32error.c win32setlocale.c: % : $(top_srcdir)/src/port/%
 	rm -f $@ && $(LN_S) $< .
 
-ip.c md5.c: % : $(backend_src)/libpq/%
+ip.c md5.c: % : $(top_srcdir)/src/common/%
 	rm -f $@ && $(LN_S) $< .
 
 encnames.c wchar.c: % : $(backend_src)/utils/mb/%

src/interfaces/libpq/fe-auth.c

@@ -38,9 +38,9 @@
 #include <pwd.h>
 #endif
 
+#include "common/md5.h"
 #include "libpq-fe.h"
 #include "fe-auth.h"
-#include "libpq/md5.h"
 
 
 #ifdef ENABLE_GSS

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