app: increase max configuration parameters length

The maximum length of box.cfg{} string parameters is now 512 instead of
256 before.

NO_DOC=no need to document
NO_TEST=will be added in EE

(cherry picked from commit f5511d45)

changelogs/unreleased/ghe-523-max-config-param-length.md

+## feature/box
+
+* The maximum length of `box.cfg{}` string parameters is now 512 instead of 256.

src/cfg.c

@@ -34,7 +34,7 @@
 #include "tt_static.h"
 #include "lua/uri.h"
 
-enum { MAX_OPT_NAME_LEN = 256, MAX_OPT_VAL_LEN = 256 };
+enum { MAX_OPT_NAME_LEN = 256, MAX_OPT_VAL_LEN = 512 };
 
 static void
 cfg_get(const char *param)