edituser.cgi: add trailing "\n" to keys form field data

edituser.cgi: add trailing "\n" to keys form field data

It's much easier to add a new key when clicking in the box of
keys below the last key sets the cursor to the first blank line
after the existing keys.

We make that happen by appending "\n" to the keys list data we
stuff into the form field.  Technically this creates a blank
line in the field at the end, but we trim blank lines before
saving so it ends up providing a better user experience.