Make sure that requested effect id is not out of range for the
device and that effect is present before requesting device to
play it.

Reported-by: Jan Kratochvil <honza@jikos.cz>
Signed-off-by: Dmitry Torokhov <dtor@mail.ru>

Add input subsystem to kernel-api docbook.
Enhance some function and parameter comments.

Signed-off-by: Randy Dunlap <randy.dunlap@oracle.com>
Signed-off-by: Dmitry Torokhov <dtor@mail.ru>

Implement a new force feedback interface, in which all non-driver-specific
operations are separated to a common module. This includes handling effect
type validations, locking, etc.

The effects are now file descriptor specific instead of the previous strange
half-process half-fd specific behaviour. The effect memory of devices is not
emptied if the root user opens and closes the device while another user is
using effects. This is a minor change and most likely no force feedback
aware programs are affected by this negatively.

Otherwise the userspace interface is left unaltered.

Signed-off-by: Anssi Hannula <anssi.hannula@gmail.com>
Signed-off-by: Dmitry Torokhov <dtor@mail.ru>