listio support through a system call(lio_submit)

This builds on the infrastructure provided by Sebastien's AIO
completion signal notification and listio patches to provide listio
support via a new system call.

long lio_submit(aio_context_t ctx_id, int mode, long nr,
	struct iocb __user * __user *iocbpp, struct sigevent __user *event)

More details about the system call appears within the patch itself.

Sebastien had taken the approach of overloading the io_submit() with
new aio_lio_opcode of IOCB_CMD_GROUP to support the listio behaviour.
And this is an alternative approach for supporting listio.

Would this system call approach be agreeable ?

Would it make sense to add another argument to support partial
completion notification ? i,e., generate a notification when a minimum
number of ios complete. Would such a feature be desirable ? Is it ok
to add one more argument to the system call for this purpose ?

This patch along with the previous 4 AIO completion signal notification
patches have been tested using libposix-aio library. Tests have been
done on x86 and x86_64 boxes. The compat syscall changes have been tested
on x86_64 system.