valid_sg_phone_number() in phone.sg.inc only validates Singapore fixed line numbers, and without country code

As the title summarises, valid_sg_phone_number() only validates Singapore fixed line numbers which start with the digit 6. Mobile numbers do not validate as they start either with 8 or 9. Numbers with the country code included e.g. +65 61234567 also do not validate.

For reference, see the Wikipedia page on Singapore phone numbers.

Attached is a patch to address these two issues.