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

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