Define IN6ADDR_ANY and IN6ADDR_LOOPBACK.

* libguile/socket.c (scm_init_socket): Define IN6ADDR_ANY and IN6ADDR_LOOPBACK.
* doc/ref/posix.texi (Network Address Conversion): Document them.

doc/ref/posix.texi

@@ -2524,7 +2524,22 @@ Make an IPv4 Internet address by combining the network number
 @cindex IPv6
 
 An IPv6 Internet address is a 16-byte value, represented in Guile as
-an integer in host byte order, so that say ``::1'' is 1.
+an integer in host byte order, so that say ``::1'' is 1.  The following
+constants are defined for convenience.
+
+@defvar IN6ADDR_ANY
+For a server, this can be used with @code{bind} (@pxref{Network Sockets
+and Communication}) to allow connections from any IPv6 interface on the
+machine.
+@end defvar
+
+@defvar IN6ADDR_LOOPBACK
+The address of the local host using the loopback device, ie.@:
+@samp{::1}.
+@end defvar
+
+The procedures below convert an IPv6 @emph{or} an IPv4 address to and
+from its textual representation.
 
 @deffn {Scheme Procedure} inet-ntop family address
 @deffnx {C Function} scm_inet_ntop (family, address)

libguile/socket.c

@@ -1656,6 +1656,7 @@ scm_init_socket ()
   /* standard addresses.  */
 #ifdef INADDR_ANY
   scm_c_define ("INADDR_ANY", scm_from_ulong (INADDR_ANY));
+  scm_c_define ("IN6ADDR_ANY", scm_from_ulong (0));
 #endif
 #ifdef INADDR_BROADCAST
   scm_c_define ("INADDR_BROADCAST", scm_from_ulong (INADDR_BROADCAST));
@@ -1665,6 +1666,7 @@ scm_init_socket ()
 #endif
 #ifdef INADDR_LOOPBACK
   scm_c_define ("INADDR_LOOPBACK", scm_from_ulong (INADDR_LOOPBACK));
+  scm_c_define ("IN6ADDR_LOOPBACK", scm_from_ulong (1));
 #endif
 
   /* socket types.