unix: add bindings for keyutils syscalls

This CL adds support for the ADD_KEY, REQUESET_KEY, and KEYCTL syscalls.
These syscalls allow access to the kernel keyring facilities. In C code,
these syscalls are wrapped by the libkeyutils package. See:

The ADD_KEY and REQUEST_KEY calls are fairly straightforward, but the
KEYCTL syscall is a multiplexor for a number of key management
functions. See:

The Go bindings for the KEYCTL functions attempt to replicate what
libkeyutils does. This is done via generated helper functions.

Change-Id: If8c97d4ef5bce14c43dee3e6772ded42dc3c595a
Reviewed-on: https://go-review.googlesource.com/41415
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
36 files changed