RE: Add partial :-variable expansion to psql \copy

Hello,

I've been biten by psql's \copy lack of variable expansion, in a limited-access docker-inside-VM context where COPY is not a viable option and hardwired names are not desirable. The attached patch allows \copy to use variable's values in place of table and file names:

```psql

\set table 'some table'

\set input 'some file name.csv'

\copy :"table" from :'input' with (format csv)

```

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