Windows: Make Rx PMTU discovery configurable; disable by default

Windows: Make Rx PMTU discovery configurable; disable by default

Disable PMTU discovery by default.  Permit it to be enabled
with the RxPMTUDiscovery TransarcAFSDaemon\Parameter value.

LICENSE MIT

Reviewed-on: http://gerrit.openafs.org/3032
Reviewed-by: Jeffrey Altman <jaltman@openafs.org>
Tested-by: Jeffrey Altman <jaltman@openafs.org>
(cherry picked from commit 2c12b80a4fdb8323ff3fdd194ff1994ad0a0b8e1)
Change-Id: Ic5b3d8e267bc10ee1802e5c391f87d37d605bec9
Reviewed-on: http://gerrit.openafs.org/3141
Tested-by: Derrick Brashear <shadow@dementia.org>
Reviewed-by: Derrick Brashear <shadow@dementia.org>