[copy.pl] add gpm copybuffer

@bw1
It might be possible to use `gpm` to copy text if in non-X env
There should be a fifo called /dev/gpmdata
```

> -R[name]
> Causes gpm to act as a repeater: any mouse data received while in graphic
> mode will be produced on the fifo /dev/gpmdata in protocol name, given as
> an optional argument (no space in between !). In principle, you can use the
> same names as for the -t option, although repeating into some protocols may
> not be implemented for a while. In addition, you can specify raw as the name,
> to repeat the mouse data byte by byte, without any protocol translation. If
> name is omitted, it defaults to msc

```
Don't know without testing myself if it's possible to write data to gpmdata and paste with mouse middle-button, also, no idea if gpm has to be started with -R for gpmdata to be created
Something to test :)