Attempt to open the device for reading before actually adding the device to the session list.  If the device comes back as unconfigured, just ignore that line in /etc/ttys.  If someone HUP's init, we'll try again.
Attempt to open the device for reading before actually adding the device
to the session list.  If the device comes back as unconfigured, just
ignore that line in /etc/ttys.  If someone HUP's init, we'll try again.

This change stops getty's from hanging on vty and sio ports that don't
exist, either due to LKM drivers not being loaded, or probes failing.
Reviewed by:	bde

jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase