console: add /proc/consoles

It allows users to see what consoles are currently known to the system
and with what flags.

It is based on Werner's patch, the part about traversing fds was
removed, the code was moved to kernel/printk.c, where consoles are
handled and it makes more sense to me.

Signed-off-by: Jiri Slaby <jslaby@suse.cz> [cleanups]
Signed-off-by: "Dr. Werner Fink" <werner@suse.de>
Cc: Al Viro <viro@ZenIV.linux.org.uk>
Cc: Greg Kroah-Hartman <gregkh@suse.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>