syslets: add indirect args

syslets: add indirect args

This adds the syslet indirect args to the indirect_params union.

This is broken, but it lets us simply demonstrate the rest of the syslet
universe around the indirect argument passing convention.

A caller could well want to perform a syscall that uses indirect arguments as a
syscall.  Maybe we turn indirect_params into a struct that contains a union for
arguments which can never be used concurrently.  This needs wider discussion.

Signed-off-by: Zach Brown <zach.brown@oracle.com>
Signed-off-by: Jens Axboe <axboe@axboetest.dk.oracle.com>