pgsql: Add pgtcl back to the list of externally-maintained client inter

Add pgtcl back to the list of externally-maintained client interfaces.

FlightAware is still maintaining this, and indeed is seemingly being
more active with it than the pgtclng fork is. List both, for the
time being anyway.

In the back branches, also back-port commit e20f679f6 and other
recent updates to the client-interfaces list. I think these are
probably of current interest to users of back branches. I did
not touch the list of externally maintained PLs in the back
branches, though. Those are much more likely to be server version
sensitive, and I don't know which of these PLs work all the way back.

Discussion: https://postgr.es/m/[email protected]

Branch
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REL9_3_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/fb31e3a272e7b18e6f86b6b58f0833c5726322b6

Modified Files
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doc/src/sgml/external-projects.sgml | 35 ++++++++++++++++++++++++++++-------
1 file changed, 28 insertions(+), 7 deletions(-)