unix: add flags parameter to Unlinkat

The flags parameter was added in https://golang.org/cl/7917.  In that CL
the reviewers discussed adding UnlinkAt and changing Unlinkat, and opted
for the latter.

Unfortunately, I then undid that in https://golang.org/cl/10032, which
was a backport of (among others) https://golang.org/cl/5837.  I didn't
notice that in the syscall package 5837 removed Unlinkat with two
parameters, but the port to the x/sys/unix package removed Unlinkat with
three parameters.  Argh.

This CL effectively restores 7917, by adding the parameter back.

Fixes golang/go#9923.

Change-Id: I8fe218ac637d0eb0346b63b596666671374bd19f
Reviewed-on: https://go-review.googlesource.com/18981
Reviewed-by: Rob Pike <r@golang.org>