Rethink \crosstabview's argument parsing logic.

Rethink \crosstabview's argument parsing logic.

\crosstabview interpreted its arguments in an unusual way, including
doing case-insensitive matching of unquoted column names, which is
surely not the right thing.  Rip that out in favor of doing something
equivalent to the dequoting/case-folding rules used by other psql
commands.  To keep it simple, change the syntax so that the optional
sort column is specified as a separate argument, instead of the
also-quite-unusual syntax that attached it to the colH argument with
a colon.

Also, rework the error messages to be closer to project style.