lib:util: codepoint_cmpi: be transitive and case-insensitive

lib:util: codepoint_cmpi: be transitive and case-insensitive

the less/greater conparisons were not case-sensitive, which made the whole
function non-transitive.

I think codepoint_cmpi() is currently only used for equality tests, so
nothing will change.

Signed-off-by: Douglas Bagnall <douglas.bagnall@catalyst.net.nz>
Reviewed-by: Andrew Bartlett <abartlet@samba.org>