Pledge, the privilege-restricting syscall and mitigation mechanism,
was missing from syscall_openbsd.go. As of the latest release, it
is officially supported in "stable".

More information about the call itself, and hence its importance,
can be found at:

http://www.openbsd.org/papers/hackfest2015-pledge/mgp00001.html

Change-Id: I2fdac1968664668e7bea1175677efe6433e0125e
Reviewed-on: https://go-review.googlesource.com/21815
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>