libthr: thr_attr.c: EINVAL, not ENOTSUP, on invalid arguments

On first read, POSIX may seem ambiguous about the return code for some
scheduling-related pthread functions on invalid arguments. But a more
thorough reading and a bit of standards archeology strongly suggests
that this case should be handled by EINVAL and that ENOTSUP is reserved
for implementations providing only part of the functionality required by
the POSIX option POSIX_PRIORITY_SCHEDULING (e.g., if an implementation
doesn't support SCHED_FIFO, it should return ENOTSUP on a call to, e.g.,
sched_setscheduler() with 'policy' SCHED_FIFO).

This reading is supported by the second sentence of the very definition
of ENOTSUP, as worded in CAE/XSI Issue 5 and POSIX Issue 6: "The
implementation does not support this feature of the Realtime Feature
Group.", and the fact that an additional ENOTSUP case was added to
pthread_setschedparam() in Issue 6, which introduces SCHED_SPORADIC,
saying that pthread_setschedparam() may return it when attempting to
dynamically switch to SCHED_SPORADIC on systems that doesn't support
that.

glibc, illumos and NetBSD also support that reading by always returning
EINVAL, and OpenBSD as well, since it always returns EINVAL but the
corresponding code has a comment suggesting returning ENOTSUP for
SCHED_FIFO and SCHED_RR, which it effectively doesn't support.

Additionally, always returning EINVAL fixes inconsistencies where EINVAL
would be returned on some out-of-range values and ENOTSUP on others.

Reviewed by: markj
Approved by: markj (mentor)
MFC after: 2 weeks
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D43006