Bug 3393

We have a large shared ssh config that is shared between many
company employees.  A few of the entries have items like

    Host bla+*
        ProxyCommand ssh -q bla-jumpbox nc $(sed 's@bla+@@; s@.blahnet@@'<<<%h).blahnet %p


Which usually works perfectly.  Today I learned that some users use
shells where this is not valid syntax (fish).

As a temporary workaround I advised the user to try to alias `ssh`
and `scp` so that they are prefixed with `SHELL=/bin/sh`.

Obviously we would make all of these wrapped with `sh -c "...."`
but that will make the configs less readable and we also would 
probably miss some because the user adding an entry is most likely
the rare `fish` user.

Suggestion: Add an ssh_config keyword such as `LocalShell` to 
override current logic when determining local shell to use
when executing:
    - ProxyCommand,
    - LocalCommand, &
    - Match exec statements

NOTE: this is about the *LOCAL* (client side) shell.