Added support for PuTTY on Windows

For PuTTY, use
MONITOR_CMD=putty

The optional parameter MONITOR_PARMS can be used as well, which will be passed to PuTTY, eg.
MONITOR_PARMS=8,1,n,N

Please note that this changes the default behaviour of get_monitor_port under Windows (which technically isn't needed for PuTTY support).
Tested on Windows under MSYS, NOT on Windows under Cygwin or *nix