Whitespace fix.
Separate out logic for determining if a GPG option is allowed.

 * Add function _is_allowed().
 * Update documentation for _is_allowed() and _sanitise().
Update documentation for function _sanitise().
Update documentation for function _is_allowed().
Add calls to _underscore() in _is_allowed() to avoid repeating the call in
other functions.
Rewrite function _sanitise() with calls to other new check functions().

 * Remove function _type_check_and_remove_escapes().
Update documentation for function _sanitise().