unix: add Getrandom syscall bindings and flags

This CL introduces the Getrandom syscall. Allowing users of
sufficiently new kernels (3.17) to detect when the operating system
lacks enough entropy to generate cryptographic random data. See:
http://man7.org/linux/man-pages/man2/getrandom.2.html

As we have updated the syscall numbers, the Getrandom function can
just be generated normally. The only other change is to mkerrors.sh
to include the GRND_* flags for getrandom from <linux/random.h>.

It is important to note that currently the mkerrors.sh script cannot
actually be run as it has fallen behind the 4.10 kernel. The GRND_*
flags were added manually.

This was originally part of a larger commit that was split up.
See CL: https://go-review.googlesource.com/37570

Change-Id: I464088a9c9f7224abdcdae476b30a4f2f412d9f5
Reviewed-on: https://go-review.googlesource.com/37589
Reviewed-by: Matt Layher <mdlayher@gmail.com>
Reviewed-by: Ian Lance Taylor <iant@golang.org>
Run-TryBot: Matt Layher <mdlayher@gmail.com>
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

unix/syscall_linux.go

@@ -1038,6 +1038,7 @@ func Getpgrp() (pid int) {
 //sysnb	Getpid() (pid int)
 //sysnb	Getppid() (ppid int)
 //sys	Getpriority(which int, who int) (prio int, err error)
+//sys	Getrandom(buf []byte, flags int) (n int, err error)
 //sysnb	Getrusage(who int, rusage *Rusage) (err error)
 //sysnb	Getsid(pid int) (sid int, err error)
 //sysnb	Gettid() (tid int)