[psql] Setting the PROMPT on command line

Hi there,

I wrote a tiny patch to prevent psql from overwriting the PROMPT[1-3]
variables with default values after parsing the command line. That way
the prompt can be set from the command line by using:

psql -v PROMPT1=...

The patch is against postgres 7.4.5, but it's so trivial it should be
possible to apply it to CVS without changes.

        Cheers,
            Ingo