Implemented the SCAN_UNREAD mode bit for scansubs(), scanallsubs(),

scanposts(), and listsub(). These functions are exposed in the JS 'bbs' object
as scan_subs(), scan_msgs(), and list_msgs().
Wherease previously, all of "your messages" (messages addressed to you)
would be found/displayed if the user answered (Y)es to the "Subjects Only?"
prompt, and only un-read message (to you) would be displayed if they answered
(N)o to the "Subjects Only?", providing long-standing confusion about the
different behavior, now the user is prompted whether to display un-read
messages only (in scansubs/scanallsubs), and in the case of the logon
question to "Search all subs for un-read messages to you?", only un-read
messages are listed/displayed, regardless of how the user answers the
"Subjects Only?" question.
SCAN_UNREAD is only applicable when combined with SCAN_TOYOU.

Added new commands to the msg reading prompt: 'N' (list new messages), 'U'
list un-read messages to-you.

2 new text.dat entries: DisplayUnreadMessagesOnlyQ and NoMessagesFound