Use sort.Search to look up sysctl mib values from names instead of using
a hand-crafted binary search.

Also add a simple test to verify sysctl is still working after the
change.

Change-Id: I844840ac434a704b14c3166c3730daffabd2f347
Reviewed-on: https://go-review.googlesource.com/76930
Run-TryBot: Tobias Klauser <tobias.klauser@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>