s390/hvc_iucv: Automatically assign free HVC terminal devices

Add the generic "lnxhvc" terminal ID to automatically assign a HVC terminal when
connecting to the HVC IUCV terminal device driver.  The terminal device driver
tries to find a free (not connected) HVC terminal to satisfy the incoming
connection request.

With this improvement, you do not longer need to guess which HVC terminal is
free, that is, not connected.  Also you can still connect to a particular HVC
terminal by using its associated terminal ID.

Signed-off-by: Hendrik Brueckner <brueckner@linux.vnet.ibm.com>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>