unix: add *at functions (POSIX.1-2008) on FreeBSD

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/52070
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
diff --git a/unix/ztypes_freebsd_amd64.go b/unix/ztypes_freebsd_amd64.go
index 091c10a..d987a00 100644
--- a/unix/ztypes_freebsd_amd64.go
+++ b/unix/ztypes_freebsd_amd64.go
@@ -505,6 +505,13 @@
 	Ospeed uint32
 }
 
+const (
+	AT_FDCWD            = -0x64
+	AT_REMOVEDIR        = 0x800
+	AT_SYMLINK_FOLLOW   = 0x400
+	AT_SYMLINK_NOFOLLOW = 0x200
+)
+
 type CapRights struct {
 	Rights [2]uint64
 }