[Bluetooth] Store remote modem status for RFCOMM TTY

When switching a RFCOMM socket to a TTY, the remote modem status might
be needed later. Currently it is lost since the original configuration
is done via the socket interface. So store the modem status and reply
it when the socket has been converted to a TTY.

Signed-off-by: Denis Kenzior <denis.kenzior@trolltech.com>
Signed-off-by: Marcel Holtmann <marcel@holtmann.org>