Improve psql's \d command to show whether index columns are key columns.

Improve psql's \d command to show whether index columns are key columns.

This is essential information when looking at an index that has
"included" columns.  Per discussion, follow the style used in \dC
and some other places: column header is "Key?" and values are "yes"
or "no" (all translatable).

While at it, revise describeOneTableDetails to be a bit more maintainable:
avoid hard-wired column numbers and multiple repetitions of what needs
to be identical test logic.  This also results in the emitted catalog
query corresponding more closely to what we print, which should be a
benefit to users of ECHO_HIDDEN mode, and perhaps a bit faster too
(the old logic sometimes asked for values it would not print, even
ones that are fairly expensive to get).

Discussion: https://postgr.es/m/21724.1531943735@sss.pgh.pa.us