Remove some explicit sbrk() usage

Mac OS X 10.9 now considers this function deprecated and warns on
its use, causing the buildslave configuration to error out.

Use the library routine to get a process's size instead of inlining
the call to sbrk (which is unlikely to have worked as intended for
quite some time -- most malloc implementations in use do not use
sbrk to get their storage).

Change-Id: If616e1ebbea7c0aa541fb96c486820e883363df1
Reviewed-on: http://gerrit.openafs.org/10696
Reviewed-by: Derrick Brashear <shadow@your-file-system.com>
Tested-by: BuildBot <buildbot@rampaginggeek.com>