add option to rxperf to use rx_Readv() instead of rx_Read()

add option to rxperf to use rx_Readv() instead of rx_Read()

rx_Readv() is a bit "faster" than rx_Read() and typically used by most
afs transaction.

server% rxperf server
client% rxperf client -c send -s server
SEND: threads   1, times        100, bytes      1048576:            2073 msec

server% rxperf server -v
client% rxperf client -c send -s server
SEND: threads   1, times        100, bytes      1048576:             983 msec

Reviewed-on: http://gerrit.openafs.org/2877
Reviewed-by: Derrick Brashear <shadow@dementia.org>
Tested-by: Derrick Brashear <shadow@dementia.org>
(cherry picked from commit 12e6531f0079597280e1b3961738af55d682d3f5)
Change-Id: Ia54818dddd693dfe5b41e8459f55453dd1c8361f
Reviewed-on: http://gerrit.openafs.org/3040