Allow bhyve and bhyveload to attach to tty devices.

bhyveload: introduce the -c <device> parameter
 to select a tty for output (or "stdio")

bhyve: allow the puc and lpc-com backends to
 accept a tty in addition to "stdio"

When used in conjunction with the null-modem device,
nmdm(4), this allows attach/detach to the guest console
and multiple concurrent serial ports. kgdb on a serial
port is now functional.

Reviewed by:	neel
Requested by:	Almost everyone that has used bhyve
MFC after:	10.0