MINOR: export localpeer as an environment variable

Export localpeer as the environment variable $HAPROXY_LOCALPEER,
allowing to use this variable in the configuration file.

It's useful to use this variable in the case of synchronized
configuration between peers.

doc/configuration.txt

@@ -476,6 +476,9 @@ underscore ("_") but should not start with a digit.
 
       user "$HAPROXY_USER"
 
+A special variable $HAPROXY_LOCALPEER is defined at the startup of the process
+which contains the name of the local peer. (See "-L" in the management guide.)
+
 
 2.4. Time format
 ----------------

doc/management.txt

@@ -161,7 +161,9 @@ list of options is :
     that a faulty configuration doesn't prevent the system from booting.
 
   -L <name> : change the local peer name to <name>, which defaults to the local
-    hostname. This is used only with peers replication.
+    hostname. This is used only with peers replication. You can use the
+    variable $HAPROXY_LOCALPEER in the configuration file to reference the
+    peer name.
 
   -N <limit> : sets the default per-proxy maxconn to <limit> instead of the
     builtin default value (usually 2000). Only useful for debugging.

src/haproxy.c

@@ -1293,6 +1293,7 @@ static void init(int argc, char **argv)
 	gethostname(hostname, sizeof(hostname) - 1);
 	memset(localpeer, 0, sizeof(localpeer));
 	memcpy(localpeer, hostname, (sizeof(hostname) > sizeof(localpeer) ? sizeof(localpeer) : sizeof(hostname)) - 1);
+	setenv("HAPROXY_LOCALPEER", localpeer, 1);
 
 	/*
 	 * Initialize the previously static variables.
@@ -1496,7 +1497,10 @@ static void init(int argc, char **argv)
 				case 'n' : cfg_maxconn = atol(*argv); break;
 				case 'm' : global.rlimit_memmax_all = atol(*argv); break;
 				case 'N' : cfg_maxpconn = atol(*argv); break;
-				case 'L' : strncpy(localpeer, *argv, sizeof(localpeer) - 1); break;
+				case 'L' :
+					strncpy(localpeer, *argv, sizeof(localpeer) - 1);
+					setenv("HAPROXY_LOCALPEER", localpeer, 1);
+					break;
 				case 'f' :
 					if (!list_append_word(&cfg_cfgfiles, *argv, &err_msg)) {
 						ha_alert("Cannot load configuration file/directory %s : %s\n",