Implement "-f" for "--copy"

[certik]

• an update --copy that would overwrite an existing profile copy instead
  of
  failing. Currently you have to do 'chmod -R u+rwX my_profile; rm -rf
  my_profile; update --copy myprofile'.

I don't know if overwriting it is a good idea, as you loose all user
compiled programs. So I think that

./update --copy xx

should fail if the "xx" directory exists. You would have to do:

./update -f --copy xx

to force remove it and reinstall it. Btw, I think that only "chmod -R
+w" is enough.