sched: enhance decoding of sched_setattr syscall

sched: enhance decoding of sched_setattr syscall

Implement read/write semantics of struct sched_attr.size argument
of sched_setattr syscall.  Do not print members of struct sched_attr
besides sched_attr.size when the specified structure size is less than
the minimal size allowed by the kernel.

* sched.c (print_sched_attr): Fetch struct sched_attr.size and use it
as the structure size.  Print struct sched_attr.size only when the
structure size is less than SCHED_ATTR_MIN_SIZE.
(SYS_FUNC(sched_setattr)): Call print_sched_attr with zero size
argument.  Print struct sched_attr.size returned by the kernel
on exiting syscall in case of E2BIG. Print the last syscall argument
on exiting syscall.
(SYS_FUNC(sched_getattr)): Do not call print_sched_attr with zero size
argument.
* NEWS: Mention it.
* tests/sched_xetattr.c (main): Check it.