Fix format_type() to restore its old behavior.

Fix format_type() to restore its old behavior.

Commit a26116c6c accidentally changed the behavior of the SQL format_type()
function while refactoring.  For the reasons explained in that function's
comment, a NULL typemod argument should behave differently from a -1
argument.  Since we've managed to break this, add a regression test
memorializing the intended behavior.

In passing, be consistent about the type of the "flags" parameter.

Noted by Rushabh Lathia, though I revised the patch some more.

Discussion: https://postgr.es/m/CAGPqQf3RB2q-d2Awp_-x-Ur6aOxTUwnApt-vm-iTtceZxYnePg@mail.gmail.com