Fix: can_* and is_* properties were not dynamic. Added "posts" property.

The following properties would only reflect the status at the time the
msg_area object was initialized:
- can_access
- can_read
- can_post
- is_operator
- is_moderated

... so if changes were made the user while online, for example, these
property values would *not* also change to reflect the current state.

The grp[] and grp_list[] 'can_access' properties still have this flaw.

Also: Add a new "posts" property to the sub[] and sub_list[] objects to
report the current number of posted messages (quicker than opening a MsgBase
instance).