Add the Faccessat, Fchmodat, Fchownat, Linkat, Mkdirat, Readlinkat,
Renameat, Symlinkat and Unlinkat functions (to conform to POSIX.1-2008)
and also add the missing AT_* constants.

Generated on FreeBSD 10.11-RELEASE for freebsd/amd64 and freebsd/386.

Change-Id: I1aea155d5375cfd6b38662018ccb5b834151b982
Reviewed-on: https://go-review.googlesource.com/52070Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>