afscp: Add -s option

Add an -s option to afscp, to specify an amount of time to sleep in
the middle of a read or write operation. This can be helpful in
simulating a slow client.

Reviewed-on: http://gerrit.openafs.org/3291
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Derrick Brashear <shadow@dementia.org>
(cherry picked from commit 00023f168e0e097c6a7e9da4450a009e4818a1a5)

Change-Id: Ifefce12d7372f9f8fe7149b91af0d0a7f128cb96
Reviewed-on: http://gerrit.openafs.org/9416
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Derrick Brashear <shadow@your-file-system.com>
Reviewed-by: Andrew Deason <adeason@sinenomine.net>
Reviewed-by: Stephan Wiesand <stephan.wiesand@desy.de>