This adds a new commandline option --read-only (or -r). It's implemented
according to the behavior agreed on on the mailing list:

--read-only applies to all files on the command line
irrespective of positioning and has no effect on any other files
opened by session or menu (...)

Current behaviour on attempting to re-open a file with different
read-only status is that nothing happens, the already open
file is raised but not changed. (...)