Fix lquery parsing to handle repeated flag characters correctly,
and to enforce the max label length correctly in some cases where
it did not before, and to detect empty labels in some cases where
it did not before.

In a more cosmetic vein, use a switch rather than if-then chains to
handle the different states, and avoid unnecessary checks on charlen
when looking for ASCII characters, and factor out multiple copies of
the label length checking code.

Tom Lane and Dmitry Belyavsky

Discussion: https://postgr.es/m/CADqLbzLVkBuPX0812o+z=c3i6honszsZZ6VQOSKR3VPbB56P3w@mail.gmail.com