RX: No userspace atomic_ops in Solaris pre-10

RX: No userspace atomic_ops in Solaris pre-10

The atomic_ops(3C) family of functions do not exist in userspace
before Solaris 10. So, only use them for rx_atomic operations if
we're on AFS_SUN510_ENV, or if we're in the kernel.

Also, include <sys/atomic.h> for kernel code and <atomic.h> for
userspace. Although they are currently equivalent, they are documented
as separate, and pre-10 lacks <atomic.h>.

Change-Id: Id71857efcd6460f8223302850b548754c19d076f
Reviewed-on: http://gerrit.openafs.org/3652
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Derrick Brashear <shadow@dementia.org>