module_format_fun_env fixes

module_format_fun_env fixes

* src/doprnt.c (exprintf) [HAVE_MODULES]: Also define in this case.
* src/emacs-module.c (module_format_fun_env):
Convert path and sym to UTF-8.
Don’t use VLAs, as the C11 standard says they’re optional,
and anyway they can cause core dumps with large allocations.
Use exprintf rather than snprintf, as exprintf handles arbitrarily
long strings.  Simplify the code a bit.