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.
(cherry picked from commit
ad752b80d7fa064b7bdd3d4c8d47c95d79265b58)