syscall: change SysProcAttr.Token type to Token

syscall: change SysProcAttr.Token type to Token

CL 75253 introduced new SysProcAttr.Token field as Handle.
But we already have exact type for it - Token. Use Token
instead of Handle everywhere - it saves few type conversions
and provides better documentation for new API.

Change-Id: Ibc5407a234a1f49804de15a24b27c8e6a6eba7e0
Reviewed-on: https://go-review.googlesource.com/76314
Reviewed-by: Ian Lance Taylor <iant@golang.org>