allow CSV quote in NULL

Greetings,

Please find attached a minor patch to remove the constraints that a
user can't include the delimiter or quote characters in a 'NULL AS'
string when importing CSV files.

This allows a user to explicitly request that NULL conversion happen
on fields which are quoted. As the quote character is being allowed
to be in the 'NULL AS' string now, there's no reason to exclude the
delimiter character from being seen in that string as well, though
unless quoted using the CSV quote character it won't ever be matched.

An example of the usage:

sfrost*=# \copy billing_data from ~/BillingSamplePricerFile.csv
with csv header quote as '"' null as '""'

This is no contrived example, it's an issue I ran into earlier today
when I got a file which had (for reasons unknown to me and not easily
changed upstream):

"1","V","WASHDCABC12","","120033"...

Both of the ending columns shown are integer fields, the "" here being
used to indicate a NULL value.

Without the patch, an ERROR occurs:

sfrost=> \copy billing_data from ~/BillingSamplePricerFile.csv
with csv header quote as '"'
ERROR: invalid input syntax for integer: ""

And there's no way to get it to import with COPY CSV mode. The
patch adds this ability without affecting existing usage or changing
the syntax. Even with the patch an ERROR occurs with the default
treatment of CSV files:

sfrost=# \copy billing_data from ~/BillingSamplePricerFile.csv
with csv header quote as '"'
ERROR: invalid input syntax for integer: ""

Which would be expected. If the file is modified to remove the ""s
for NULL columns, it imports just fine with the syntax above.

It'd be really nice to have this included.

Thanks!

Stephen