Adjust keybindings to also work in mc subshell

mc is doing some weird stuff when (trying) to open the subshell,
which breaks with the keybindings we use to insert head|less|grep
and our magic-parentheses.

so we now leave them out when doing subshell in mc - detected by
the env variable MC_SID being set. thats not perfect, but meh, better
than no subshell at all.
just use the shortcut zsh has