bug#51850: 27.2; No way to configure mm-external-terminal-program command switch

[Kevin Foley]

When calling `mm-display-external', the switch to pass a command to
`mm-external-terminal-program' is hard coded to "-e".

However this switch is deprecated in `gnome-terminal` and "--" should be
used instead.  In my testing "-e" didn't work at all with
`gnome-terminal` (I tested the same command outside of Emacs as well).

I think it would be useful to make this configurable via a user option.

Thanks,
Kevin Foley