net.box: allow calling stored Lua and C module functions with self.call

The fix is simple: look up the function in `box.func` by name and, if
found, execute its `call` method. The only tricky part is to avoid the
lookup before `box.cfg` is called because `box.func` is unavailable at
the time. We achieve that by checking `box.ctl.is_recovery_finished`.

Closes #9131

NO_DOC=bug fix