Fix assorted issues in client host name lookup.

The code for matching clients to pg_hba.conf lines that specify host names
(instead of IP address ranges) failed to complain if reverse DNS lookup
failed; instead it silently didn't match, so that you might end up getting
a surprising "no pg_hba.conf entry for ..." error, as seen in bug #9518
from Mike Blackwell.  Since we don't want to make this a fatal error in
situations where pg_hba.conf contains a mixture of host names and IP
addresses (clients matching one of the numeric entries should not have to
have rDNS data), remember the lookup failure and mention it as DETAIL if
we get to "no pg_hba.conf entry".  Apply the same approach to forward-DNS
lookup failures, too, rather than treating them as immediate hard errors.

Along the way, fix a couple of bugs that prevented us from detecting an
rDNS lookup error reliably, and make sure that we make only one rDNS lookup
attempt; formerly, if the lookup attempt failed, the code would try again
for each host name entry in pg_hba.conf.  Since more or less the whole
point of this design is to ensure there's only one lookup attempt not one
per entry, the latter point represents a performance bug that seems
sufficient justification for back-patching.

Also, adjust src/port/getaddrinfo.c so that it plays as well as it can
with this code.  Which is not all that well, since it does not have actual
support for rDNS lookup, but at least it should return the expected (and
required by spec) error codes so that the main code correctly perceives the
lack of functionality as a lookup failure.  It's unlikely that PG is still
being used in production on any machines that require our getaddrinfo.c,
so I'm not excited about working harder than this.

To keep the code in the various branches similar, this includes
back-patching commits c424d0d105 and
1997f34db4 into 9.2 and earlier.

Back-patch to 9.1 where the facility for hostnames in pg_hba.conf was
introduced.

src/include/getaddrinfo.h

@@ -82,6 +82,9 @@
 #ifndef NI_NUMERICSERV
 #define NI_NUMERICSERV	2
 #endif
+#ifndef NI_NAMEREQD
+#define NI_NAMEREQD		4
+#endif
 
 #ifndef NI_MAXHOST
 #define NI_MAXHOST	1025

src/include/libpq/libpq-be.h

@@ -99,6 +99,20 @@ typedef struct
  * still available when a backend is running (see MyProcPort).	The data
  * it points to must also be malloc'd, or else palloc'd in TopMemoryContext,
  * so that it survives into PostgresMain execution!
+ *
+ * remote_hostname is set if we did a successful reverse lookup of the
+ * client's IP address during connection setup.
+ * remote_hostname_resolv tracks the state of hostname verification:
+ *	+1 = remote_hostname is known to resolve to client's IP address
+ *	-1 = remote_hostname is known NOT to resolve to client's IP address
+ *	 0 = we have not done the forward DNS lookup yet
+ *	-2 = there was an error in name resolution
+ * If reverse lookup of the client IP address fails, remote_hostname will be
+ * left NULL while remote_hostname_resolv is set to -2.  If reverse lookup
+ * succeeds but forward lookup fails, remote_hostname_resolv is also set to -2
+ * (the case is distinguishable because remote_hostname isn't NULL).  In
+ * either of the -2 cases, remote_hostname_errcode saves the lookup return
+ * code for possible later use with gai_strerror.
  */
 
 typedef struct Port
@@ -111,12 +125,7 @@ typedef struct Port
 	char	   *remote_host;	/* name (or ip addr) of remote host */
 	char	   *remote_hostname;/* name (not ip addr) of remote host, if
 								 * available */
-	int			remote_hostname_resolv; /* +1 = remote_hostname is known to
-										 * resolve to client's IP address; -1
-										 * = remote_hostname is known NOT to
-										 * resolve to client's IP address; 0 =
-										 * we have not done the forward DNS
-										 * lookup yet */
+	int			remote_hostname_resolv; /* see above */
 	char	   *remote_port;	/* text rep of remote port */
 	CAC_state	canAcceptConnections;	/* postmaster connection status */
 
@@ -178,6 +187,9 @@ typedef struct Port
 	char	   *peer_cn;
 	unsigned long count;
 #endif
+
+	/* This field will be in a saner place in 9.4 and up */
+	int			remote_hostname_errcode;		/* see above */
 } Port;