pgsql: Clean up parsing of ltree and lquery some more.

Clean up parsing of ltree and lquery some more.

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

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/17ca067995114ee40749d9138ba85fdd68518052

Modified Files
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contrib/ltree/expected/ltree.out | 98 ++++++++
contrib/ltree/ltree_io.c | 515 ++++++++++++++++++---------------------
contrib/ltree/sql/ltree.sql | 27 ++
3 files changed, 362 insertions(+), 278 deletions(-)