datapath-windows: refactor BuildReplyMsgFromMsgIn & BuildErrorMsg

In this patch, we consolidate code in Netlink.c.

Signed-off-by: Nithin Raju <nithin@vmware.com>
Acked-by: Eitan Eliahu <eliahue@vmware.com>
Signed-off-by: Ben Pfaff <blp@nicira.com>
datapath-windows: return bool from NlFillOvs[Msg/Hdr]

Per review comment, in this patch, we update the return values of
NlFillOvsMsg() and NlFillOvsHdr() from NTSTATUS to BOOLEAN to make them
consistent with the Nl* functions.

Signed-off-by: Nithin Raju <nithin@vmware.com>
Acked-by: Eitan Eliahu <eliahue@vmware.com>
Signed-off-by: Ben Pfaff <blp@nicira.com>