vcs: add poll/fasync support

The /dev/vcs* devices are used, amongst other things, by accessibility
applications such as BRLTTY to display the screen content onto refreshable
braille displays.  Currently this is performed by constantly reading from
/dev/vcsa0 whether or not the screen content has changed.  Given the
default braille refresh rate of 25 times per second, this easily qualifies
as the biggest source of wake-up events preventing laptops from entering
deeper power saving states.

To avoid this periodic polling, let's add support for select()/poll() and
SIGIO with the /dev/vcs* devices.  The implemented semantic is to report
data availability whenever the corresponding vt has seen some update after
the last read() operation.  The application still has to lseek() back
as usual in order to read() the new data.

Not to create unwanted overhead, the needed data structure is allocated
and the vt notification callback is registered only when the poll or
fasync method is invoked for the first time per file instance.

Signed-off-by: Nicolas Pitre <nicolas.pitre@canonical.com>
Cc: Andrew Morton <akpm@linux-foundation.org>
Acked-by: Alan Cox <alan@linux.intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>