Add a routine to get the size of the current process

Use rusage when available, and fall back to the very old sbrk(0)
hack otherwise.

Change-Id: Ic986fd6b93476b80008908a95f8b8e0c76d9ed9a
Reviewed-on: http://gerrit.openafs.org/10695
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Derrick Brashear <shadow@your-file-system.com>