Port better to FreeBSD’s dlfunc vs dlsym

Port better to FreeBSD’s dlfunc vs dlsym

This avoids warnings when converting between void * and
function pointers, which strict C11 does not allow.
* configure.ac (dlfunc): Check for existence.
* src/dynlib.c (dlfunc) [!HAVE_DLFUNC]: New macro.
(dynlib_func): New function.
* src/dynlib.h (dynlib_function_ptr, dynlib_func): New decls.
* src/emacs-module.c (Fmodule_load): Use dynlib_func, not
dynlib_sym, for function pointers.