Reject duplicate column names in foreign key referenced-columns lists.

Such cases are disallowed by the SQL spec, and even if we wanted to allow
them, the semantics seem ambiguous: how should the FK columns be matched up
with the columns of a unique index?  (The matching could be significant in
the presence of opclasses with different notions of equality, so this issue
isn't just academic.)  However, our code did not previously reject such
cases, but instead would either fail to match to any unique index, or
generate a bizarre opclass-lookup error because of sloppy thinking in the
index-matching code.

David Rowley
Further cleanup of JSON-specific error messages.

Fix an obvious typo in json_build_object()'s complaint about invalid
number of arguments, and make the errhint a bit more sensible too.

Per discussion about how to word the improved hint, change the few places
in the documentation that refer to JSON object field names as "names" to
say "keys" instead, since that's what we've said in the vast majority of
places in the docs.  Arguably "name" is more correct, since that's the
terminology used in RFC 7159; but we're stuck with "key" in view of the
naming of json_object_keys() so let's at least be self-consistent.

I adjusted a few code comments to match this as well, and failed to
resist the temptation to clean up some odd whitespace choices in the
same area, as well as a useless duplicate PG_ARGISNULL() check.  There's
still quite a bit of code that uses the phrase "field name" in non-user-
visible ways, so I left those usages alone.
Clean up handling of unknown-type inputs in json_build_object and friends.

There's actually no need for any special case for unknown-type literals,
since we only need to push the value through its output function and
unknownout() works fine.  The code that was here was completely bizarre
anyway, and would fail outright in cases that should work, not to mention
suffering from some copy-and-paste bugs.