Now as `SYS_UNVEIL` has landed in `x/sys`, it's time to have a convenient handle for it too.

Change-Id: I61db9cc23fa9f2e10bac601959002319ea9c854b
GitHub-Last-Rev: eb9487242e1fafa06c057ef49444ea5a711382df
GitHub-Pull-Request: golang/sys#19
Reviewed-on: https://go-review.googlesource.com/c/142317Reviewed-by: Tobias Klauser <tobias.klauser@gmail.com>
Run-TryBot: Tobias Klauser <tobias.klauser@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>