Windows: Query the MTU value from the registry

Windows: Query the MTU value from the registry

The syscfg_GetIFInfo() call uses the interface info returned by
GetAdaptersAddresses().  Also manually query the MTU value in the
registry for any interfaces bound to the same adapter and use the
smallest such MTU as the value returned by syscfg_GetIFInfo().

Change-Id: I8a1d487b51f6674ff6ebbf6b1f20d37b1e278279
Reviewed-on: http://gerrit.openafs.org/1105
Tested-by: Jeffrey Altman <jaltman@openafs.org>
Reviewed-by: Jeffrey Altman <jaltman@openafs.org>