afsd: Add the -rxmaxfrags option

afsd: Add the -rxmaxfrags option

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

Change-Id: I9f0b3edd95db17f82ef8f7d1e300df4eff355172
Reviewed-on: http://gerrit.openafs.org/4899
Tested-by: Derrick Brashear <shadow@dementia.org>
Reviewed-by: Derrick Brashear <shadow@dementia.org>