rxperf: -S takes an argument

rxperf: -S takes an argument

The -S option to rxperf (which permits the maximum number of server
threads to be set) takes a parameter. Update the getopt string so
that we can give it one.

Reviewed-on: http://gerrit.openafs.org/4801
Reviewed-by: Jeffrey Altman <jaltman@openafs.org>
Tested-by: Jeffrey Altman <jaltman@openafs.org>
(cherry picked from commit 80b3648e6d8fd96bdb57d47e56b28b30e42eb303)

Change-Id: Ic1a8264d0471e9458ade11af00ed78871b390b34
Reviewed-on: http://gerrit.openafs.org/8423
Reviewed-by: Derrick Brashear <shadow@your-file-system.com>
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Tested-by: Stephan Wiesand <stephan.wiesand@desy.de>
Reviewed-by: Stephan Wiesand <stephan.wiesand@desy.de>