Commit hooks ipmi_user_priv() to _ipmi_set_user_access(). Later got extended for
'change_priv_limit_only', because of the former.
Commit removes ipmi_user_set_userpriv(), because it's not used anywhere.
Commit is a re-work of 'channel getaccess' and 'channel setaccess'. These are
using _ipmi_* now. Also, bitfields and two structs are replaced by one unified
struct.