[PATCH] remove is_member_of_role() from header, add can_set_role()

As a follow-on to Conflation of member/privs for predefined roles,
this removes is_member_of_role from the header to dissuade it's use
for privilege checking. Since SET ROLE must use membership rather than
privileges a new, explicitly named can_set_role() function is
exported.

is_member_of_role_nosuper() still exists for the following purposes:
- membership loop checking in user.c
- membership matching for pg_hba.conf in hba.c

Other uses of is_member_of_role_nosuper() should be avoided.