Let tincctl use the NETNAME environment variable if no -n option is given.

This allows administrators who frequently want to work with one tinc
network to omit the -n option. Since the NETNAME variable is set by
tincd when executing scripts, this makes it slightly easier to use
tincctl from within scripts.

src/tincctl.c

@@ -138,6 +138,12 @@ static bool parse_options(int argc, char **argv) {
 		}
 	}
 
+	if(!netname) {
+		netname = getenv("NETNAME");
+		if(netname)
+			netname = xstrdup(netname);
+	}
+
 	return true;
 }