pgsql: Fix psql's \dC command to annotate I/O conversion casts assuch.

Fix psql's \dC command to annotate I/O conversion casts as such.

A cast declared WITH INOUT was described as '(binary coercible)',
which seems pretty inaccurate; let's print '(with inout)' instead.
Per complaint from Jean-Pierre Pelletier.

This definitely seems like a bug fix, but given that it's been wrong
since 8.4 and nobody complained before, I'm hesitant to back-patch a
behavior change into stable branches.  It doesn't seem too late for
v11 though.

Discussion: https://postgr.es/m/5b887023.1c69fb81.ff96e.6a1d@mx.google.com

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/115bf1e79af0114c18d7d5675f4ba7e8bd6e11e9

Modified Files
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src/bin/psql/describe.c | 56 +++++++++++++++++++++++++++++++++----------------
1 file changed, 38 insertions(+), 18 deletions(-)