Modify escape of ^ key so it can be used as well as Ctrl+^

Fixes FS#721

This makes Ctrl+^ and ^ bindable again as different keys. We do this
by escaping single `^` as `^-`, which is not a valid control character
(unlike `^^`)

The original approach suggested in FS#721 is insufficient, it will
break bindings such as `meta-^` because Irssi is convinced that `^`
introduces a Control-key ("key combo") so it is waiting for what may
follow.