libpq: Deprecate pg_int64.

Previously we used pg_int64 in three function prototypes in libpq.  It
was added by commit 461ef73f to expose the platform-dependent type used
for int64 in the C89 era.  As of commit 962da900 it is defined as
standard int64_t, and the dust seems to have settled.

Let's just use int64_t directly in these three client-facing functions
instead of (yet) another name.  We've required C99 and thus <stdint.h>
since PostgreSQL 12, C89 and C++98 compilers are long gone, and client
applications very likely use standard types for their own 64-bit needs.
This also cleans up the obscure placement of a new #include <stdint.h>
directive in postgres_ext.h, required for the new definition.  The
typedef was hiding in there for historical reasons, but it doesn't fit
postgres_ext.h's own description of its purpose and there is no evidence
of client applications including postgres_ext.h directly to see it.

Keep a typedef marked deprecated for backward compatibility, but move it
into libpq-fe.h where it was used.

Reviewed-by: Peter Eisentraut <peter@eisentraut.org>
Discussion: https://postgr.es/m/CA%2BhUKGKn_EkNNGMY5RzMcKP%2Ba6urT4JF%3DCPhw_zHtQwjvX6P2g%40mail.gmail.com

src/test/examples/testlo64.c

@@ -12,6 +12,7 @@
  *
  *-------------------------------------------------------------------------
  */
+#include <stdint.h>
 #include <stdio.h>
 #include <stdlib.h>
 
@@ -75,7 +76,7 @@ importFile(PGconn *conn, char *filename)
 }
 
 static void
-pickout(PGconn *conn, Oid lobjId, pg_int64 start, int len)
+pickout(PGconn *conn, Oid lobjId, int64_t start, int len)
 {
 	int			lobj_fd;
 	char	   *buf;
@@ -110,7 +111,7 @@ pickout(PGconn *conn, Oid lobjId, pg_int64 start, int len)
 }
 
 static void
-overwrite(PGconn *conn, Oid lobjId, pg_int64 start, int len)
+overwrite(PGconn *conn, Oid lobjId, int64_t start, int len)
 {
 	int			lobj_fd;
 	char	   *buf;
@@ -148,7 +149,7 @@ overwrite(PGconn *conn, Oid lobjId, pg_int64 start, int len)
 }
 
 static void
-my_truncate(PGconn *conn, Oid lobjId, pg_int64 len)
+my_truncate(PGconn *conn, Oid lobjId, int64_t len)
 {
 	int			lobj_fd;