OpenBSD 4.6/4.7: Define curproc for rx_atomic.h.

For OpenBSD 4.6 and 4.7, the rx_atomic.h header ultimately resorts
to the use of the default atomic routines that rely on MUTEX
macros. Those macros require that 'curproc' be defined, which in
turn requires the presence of the 'sys/proc.h' header. This patch
inserts that header into the param.h file for those systems.

Note that subsequent versions of OpenBSD have __sync_fetch_and_add
and don't require 'curproc' as a result.

Change-Id: I5131aed60631ef285eda92180ef65845165526b1
Reviewed-on: http://gerrit.openafs.org/8925
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Derrick Brashear <shadow@your-file-system.com>