psql's \copy incompatible with :variables

Summary: \copy interprets psql's :variables as literal strings

Tested On: 8.4.4, 9.0.4

Severity: Annoyance

Steps to Reproduce:

    psql

    \set filename 'test.csv'

    \copy pg_class to :filename with csv

    \q

    ls

    :filename

Note that psql variables work perfectly fine with COPY.  It's just \copy
which seems to be misbehaving.

--Josh