Always assert DCD and DSR in bhyve's uart emulation.

The /etc/ttys entry for a serial console in FreeBSD/x86 is as follows:
ttyu0 "/usr/libexec/getty 3wire" vt100 onifconsole secure

The initial terminal type passed to getty(8) is "3wire" which sets the
CLOCAL flag. However reset(1) clears this flag and any programs that try
to open the terminal will hang waiting for DCD to be asserted.

Fix this by always asserting DCD and DSR in the emulated uart.

The following discussion on virtualization@ has more details:
https://lists.freebsd.org/pipermail/freebsd-virtualization/2015-June/003666.html

Reported by: jmg
Discussed with: grehan