GEOM: Introduce gctl_add_param() API.

Make gctl_add_param() API public, allowing more precise control over
parameter flags. Previously it was impossible to properly declare
write-only ASCII parameters, used for result reporting, they were
declared as read-write binary instead, that was not nice.

MFC after: 1 month

(cherry picked from commit 2117cdd4b4ebda7db0270c56f5a3cbdd37ec8cfc)