Renamed "go" application type to "external".

There's nothing specific to Go language.  This type of application object can
be used to run any external application that utilizes libunit API.

src/nxt_application.c

@@ -507,15 +507,15 @@ nxt_app_parse_type(u_char *p, size_t length)
     str.length = length;
     str.start = p;
 
-    if (nxt_str_eq(&str, "python", 6)) {
+    if (nxt_str_eq(&str, "external", 8) || nxt_str_eq(&str, "go", 2)) {
+        return NXT_APP_EXTERNAL;
+
+    } else if (nxt_str_eq(&str, "python", 6)) {
         return NXT_APP_PYTHON;
 
     } else if (nxt_str_eq(&str, "php", 3)) {
         return NXT_APP_PHP;
 
-    } else if (nxt_str_eq(&str, "go", 2)) {
-        return NXT_APP_GO;
-
     } else if (nxt_str_eq(&str, "perl", 4)) {
         return NXT_APP_PERL;