vtysh: track and fix file-lock use in the workaround from 2004

vtysh: track and fix file-lock use in the workaround from 2004

There's a workaround in the code from a bug from back in 2004, it ends
and re-enters config mode anytime an `exit` is done from a level below
the top-level config node (e.g., from a `router isis` node). We need to
re-enter config mode with or without a lock according to how we actually
entered it to begin with.

fixes #13920

Signed-off-by: Christian Hopps <chopps@labn.net>