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: /messages/by-id/20170730162612.1449.58796@wrigleys.postgresql.org

Branch
------
REL9_6_STABLE

Details
-------
https://git.postgresql.org/pg/commitdiff/e5c87d5e1d9cad9019e2c4e4f392ba687bc58d02

Modified Files
--------------
doc/src/sgml/external-projects.sgml | 23 ++++++++++++++++++++++-
1 file changed, 22 insertions(+), 1 deletion(-)

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