httpd: allow suffixes on /sys/lang

Allow suffixes for /sys/lang, so e.g. if LANG=sv, then:

	/sys/lang	-> /lang/sv
	/sys/lang.css	-> /lang/sv.css

The uses should be rather obvious...

esp32/max80/httpd.c

@@ -434,15 +434,18 @@ static esp_err_t httpd_get_lang(httpd_req_t *req)
 
 static esp_err_t httpd_lang_redirect(httpd_req_t *req)
 {
-    char lang_buf[sizeof("/lang/") + MAX_LANG_LEN];
-    int len = snprintf(lang_buf, sizeof lang_buf, "/lang/%s",
-		       getenv_def("LANG", fallback_language));
+    char lang_buf[sizeof req->uri + MAX_LANG_LEN];
+    int len = snprintf(lang_buf, sizeof lang_buf, "/lang/%s%s",
+		       getenv_def("LANG", fallback_language),
+		       req->uri + (sizeof("/sys/lang")-1));
 
     return httpd_send_plain(req, 302, lang_buf, len, 0, 0);
 }
 
 #define STRING_MATCHES(str, len, what) \
     (((len) == sizeof(what)-1) && !memcmp((str), (what), sizeof(what)-1))
+#define STRING_MATCHES_PREFIX(str, len, what) \
+    (((len) >= sizeof(what)-1) && !memcmp((str), (what), sizeof(what)-1))
 
 static esp_err_t httpd_sys_handler(httpd_req_t *req)
 {
@@ -463,7 +466,7 @@ static esp_err_t httpd_sys_handler(httpd_req_t *req)
 
     query = *query == '?' ? query+1 : NULL;
 
-    if (STRING_MATCHES(file, filelen, "lang"))
+    if (STRING_MATCHES_PREFIX(file, filelen, "lang"))
 	return httpd_lang_redirect(req);
 
     if (STRING_MATCHES(file, filelen, "getstatus"))