ada: add BACKLOG_MAX OS constant

BACKLOG_MAX represents the number of outstanding connections in the
socket's listen queue.

gcc/ada/ChangeLog:

	* libgnat/g-socket.adb (Listen_Socket): Change default value.
	* libgnat/g-socket.ads (Listen_Socket): Likewise.
	* s-oscons-tmplt.c (BACKLOG_MAX): New.

gcc/ada/libgnat/g-socket.adb

@@ -1958,7 +1958,7 @@ package body GNAT.Sockets is
 
    procedure Listen_Socket
      (Socket : Socket_Type;
-      Length : Natural := 15)
+      Length : Natural := SOSC.BACKLOG_MAX)
    is
       Res : constant C.int := C_Listen (C.int (Socket), C.int (Length));
    begin

gcc/ada/libgnat/g-socket.ads

@@ -1219,7 +1219,7 @@ package GNAT.Sockets is
 
    procedure Listen_Socket
      (Socket : Socket_Type;
-      Length : Natural := 15);
+      Length : Natural := SOSC.BACKLOG_MAX);
    --  To accept connections, a socket is first created with Create_Socket,
    --  a willingness to accept incoming connections and a queue Length for
    --  incoming connections are specified. Raise Socket_Error on error.

gcc/ada/s-oscons-tmplt.c

@@ -1791,6 +1791,9 @@ struct sockaddr_un {
 #define SIZEOF_sockaddr_un (sizeof (struct sockaddr_un))
 CND(SIZEOF_sockaddr_un, "struct sockaddr_un")
 
+#define BACKLOG_MAX 15
+CND(BACKLOG_MAX, "number of outstanding connections in the socket's listen queue")
+
 #define SIZEOF_fd_set (sizeof (fd_set))
 CND(SIZEOF_fd_set, "fd_set")
 CND(FD_SETSIZE, "Max fd value")