Drop "pseudo" serial devices when fd is closed

[jordemort]

This sets the modem status lines for a "pseudo" serial port to zero when the associated file descriptor is closed. This is useful when using DOSEMU2 to launch BBS doors and other programs designed to communicate over a modem; it allows such programs to detect when the other side of the connection has been closed.

Some caveats:

• There is no way to "reconnect" once the file descriptor has been closed, but I don't think there'd be any way to reopen the file descriptor from the other end anyway, so I don't think this is a problem
• I'm not sure if setting MSRs to 0 here is the right thing or if I should only drop DCD. I think the difference is between simulating "lost carrier" vs simulating "someone yanked the modem cord out of the serial port" - 0 seems to get applications to behave the way I want it to, though.

Re: #2101

[stsp]

Thanks.