Fixed broken Linux build
Add asynch I/O support for AIX platform
Fix to support multiplexer devices which call tmxr_open_master directly instead of tmxr_attach so that the resulting TMXR will be in the open multiplexer list
Fixes to:

 1) retain the buffered/unbuffered line specific details across save/restore
 2) retain modem control outgoing bits (CTS and DTR) accross line resets which may occur due to carrier/connection loss.
 3) change the behavior which sends  the prior contents of the transmit buffer on a line reconnect to only happen for buffered telnet lines.
 4) avoid crash in SHOW MULTIPLEXER output when only specific lines have been attached instead of
Addition of DUP11 simulation to all Unibus simulators (PDP11, PDP10, and all Unibus VAXen)
Added makefile support for the DUP11 device
Added logic to act on the state and transitions of the RCVEN bit of the receiver CSR.