Make collation not depend on setlocale().

Make collation not depend on setlocale().

Now that the result of pg_newlocale_from_collation() is always
non-NULL, then we can move the collate_is_c and ctype_is_c flags into
pg_locale_t. That simplifies the logic in lc_collate_is_c() and
lc_ctype_is_c(), removing the dependence on setlocale().

This commit also eliminates the multi-stage initialization of the
collation cache.

As long as we have catalog access, then it's now safe to call
pg_newlocale_from_collation() without checking lc_collate_is_c()
first.

Discussion: https://postgr.es/m/cfd9eb85-c52a-4ec9-a90e-a5e4de56e57d@eisentraut.org
Reviewed-by: Peter Eisentraut, Andreas Karlsson