useradd: Fix issues with useradd dependencies

useradd: Fix issues with useradd dependencies

If recipe A requires the useradd actions of recipe B we need to
ensure that recipe B is part of the recipe A dependancy chain. In
order to do that, we introduce USERADD_DEPENDS. This makes sure
that the do_populate_sysroot_setscene of recipe B exists for
recipe A in case of a missing TMPDIR. This requires changes made in
runqueue.py by RP.

This commit along with the runqueue fixes effects:
Bug 13419 - recipes that add users to groups cannot rely on other recipes creating those groups (when population from sstate happens)
Bug 13904 - do_prepare_recipe_sysroot: postinst-useradd-* does not run in order of dependency and sometimes fails
Bug 13279 - Make sure users/groups exist for package_write_* tasks
Bug 15084 - For some reason using of same user in two recipes does not work properly

I've included the start of self-testing for useradd by adding tests for
13419 (which ends up testing 13904, 13279, 15084 by virtue of them all
      having the same root cause)

Signed-off-by: Eilís 'pidge' Ní Fhlannagáin <pidge@baylibre.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>