afsd: Add the -rxmaxfrags option

Add an option to afsd to limit the number of fragments Rx will
send/receive, called -rxmaxfrags.

Reviewed-on: http://gerrit.openafs.org/4899
Tested-by: Derrick Brashear <shadow@dementia.org>
Reviewed-by: Derrick Brashear <shadow@dementia.org>
(cherry picked from commit ba546ecf0c6a404f2efced2e600826621876ffcf)

Change-Id: Idb93fc60b3056604c1d415ac7fc134ee672ceefe
Reviewed-on: http://gerrit.openafs.org/9430
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Mark Vitale <mvitale@sinenomine.net>
Reviewed-by: Andrew Deason <adeason@sinenomine.net>
Reviewed-by: Stephan Wiesand <stephan.wiesand@desy.de>