Make JS bbs.msg_number and smb_curmsg writable

These 2 properties, specifically, could be made writable since
1. smb_t.curmsg is a special element owned by the application, not smblib
2. a special sbbs_t.current_msg_number property could be created/used to
   underlie the bbs.msg_number when there's no open message base (by sbbs).

This might be all that's needed to resolve issue #793

The other bbs.msg_* and bbs.smb_* properties are not so easily made writable,
so hopefully that's not desired.