wmaker: fix misuse of 'user_flags' instead of 'client_flags' for window attributes

wmaker: fix misuse of 'user_flags' instead of 'client_flags' for window attributes

The structure containing the information on windows contains 2 sets of
attributes, client_flags which contains those asked by the application
through Hints (like MWM Hints and others) and user_flags which was defined
to allow the user to override them.

Unfortunately many places of the code was using the wrong structure to save
the attributes to (for example by using the WSETUFLAG macro) which was
merely ok as the user_flags have priority, but when we want to provide a
clean consistent behaviour to users, we need to get things in the right
place.

Signed-off-by: Christophe CURIS <christophe.curis@free.fr>