Set c.screen in ewmh.tag and before tags in rules.execute

The current premise is that c.screen should be the same as
awful.tag.getscreen(t).

The addition in `ewmh.tag` appears to be the important part here,
changing the order in awful.rules.execute is (maybe) only for
consistency across the codebase.

This fixes the PR from https://github.com/awesomeWM/awesome/pull/17/.