More suscinct and flexible escape char fix

To handle the possibility of the escape character being anywhere in the
key passed to `key_binding_not_set`, I've opted to use parameter
replacement to escape every escape character. This is more suscinct, and
will make sure every '\' is handled.

sensible.tmux

@@ -45,8 +45,7 @@ server_option_value_not_changed() {
 }
 
 key_binding_not_set() {
-	local key="$1"
-	if [[ ${key} == '\' ]]; then key='\\'; fi
+	local key="${1//\\/\\\\}"
 	if $(tmux list-keys | grep -q "${KEY_BINDING_REGEX}${key}[[:space:]]"); then
 		return 1
 	else