This is mostly straightforward but it does introduce an odd change to
Fchflags and adds the Mlock related functions.  These changes look
correct to me but I don't know why they weren't in the original file.

Change-Id: I1a01e075566d327a78b77e7354c9fb85b6ad1f22
Reviewed-on: https://go-review.googlesource.com/8062Reviewed-by: Minux Ma <minux@golang.org>
Reviewed-by: David Crawshaw <crawshaw@golang.org>
Reviewed-by: Rob Pike <r@golang.org>