Add a new internal function, g_set_user_dirs(), which will safely
override the values returned by g_get_user_data_dir() and friends, and
the value returned by g_get_home_dir().

This is intended to be used by unit tests, and will be hooked up to them
in a following commit.

This can be called as many times as needed by the current process. It’s
thread-safe. It does not modify the environment, so none of the changes
are propagated to any subsequently spawned subprocesses.

Signed-off-by: Philip Withnall <withnall@endlessm.com>

#538