tty: Add a new file /proc/tty/consoles

Add a new file /proc/tty/consoles to be able to determine the registered
system console lines.  If the reading process holds /dev/console open at
the regular standard input stream the active device will be marked by an
asterisk.  Show possible operations and also decode the used flags of
the listed console lines.

Signed-off-by: Werner Fink <werner@suse.de>
Cc: Alan Cox <alan@lxorguk.ukuu.org.uk>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>