(BUGFIX) Changed the discard command for rcs to 'co -f -r'

Used to be set as 'unco' which a haven't been able to find in
any rcs docs, maybe it used to be a common to create it as
script or something.

It probably means that noone has used this option for ages.
(BUGFIX) After show an error message when sn can't find the discar command, it would show a second error

I have commented out the second one as it was only useful for
sn programmers.