fish: Allow '-' prefix on command line to override exit on error (RHBZ#578407).

fish: Allow '-' prefix on command line to override exit on error (RHBZ#578407).

Allow -cmd on the command line to mean that normal exit on error
behaviour is overridden, ie. we will not exit.

This allows you to do:

  guestfish -- command : -command : command

with the second command allowing errors.  (Note that '--' is required
to stop getopt parsing -command as an option).

Also this fixes the remote case which is what the original
bug report was about.