quagga: option "-z" ("--socket <path>") added

All daemons modified to support custom path to zserv
socket.

lib: generalize a zclient connection

zclient_socket_connect added. zclient_socket and
zclient_socket_un were hidden under static expression.
"zclient_serv_path_set" modified.
diff --git a/ripngd/ripng_main.c b/ripngd/ripng_main.c
index 85209a1..f4a6244 100644
--- a/ripngd/ripng_main.c
+++ b/ripngd/ripng_main.c
@@ -47,6 +47,7 @@
   { "daemon",      no_argument,       NULL, 'd'},
   { "config_file", required_argument, NULL, 'f'},
   { "pid_file",    required_argument, NULL, 'i'},
+  { "socket",      required_argument, NULL, 'z'},
   { "dryrun",      no_argument,       NULL, 'C'},
   { "help",        no_argument,       NULL, 'h'},
   { "vty_addr",    required_argument, NULL, 'A'},
@@ -112,6 +113,7 @@
 -d, --daemon       Runs in daemon mode\n\
 -f, --config_file  Set configuration file name\n\
 -i, --pid_file     Set process identifier file name\n\
+-z, --socket       Set path of zebra socket\n\
 -A, --vty_addr     Set vty's bind address\n\
 -P, --vty_port     Set vty's port number\n\
 -r, --retain       When program terminates, retain added route by ripngd.\n\
@@ -205,7 +207,7 @@
     {
       int opt;
 
-      opt = getopt_long (argc, argv, "df:i:hA:P:u:g:vC", longopts, 0);
+      opt = getopt_long (argc, argv, "df:i:z:hA:P:u:g:vC", longopts, 0);
     
       if (opt == EOF)
 	break;
@@ -225,7 +227,10 @@
 	  break;
         case 'i':
           pid_file = optarg;
-          break; 
+          break;
+	case 'z':
+	  zclient_serv_path_set (optarg);
+	  break;
 	case 'P':
           /* Deal with atoi() returning 0 on failure, and ripngd not
              listening on ripngd port... */