syscall: define bind flags on Plan 9

These bind flags were removed by mistake in CL 2167.

Fixes #17921.

Change-Id: I1e8089dade30a212b8db0b216c8299946d924d4b
Reviewed-on: https://go-review.googlesource.com/33271
Reviewed-by: Brad Fitzpatrick <[email protected]>
Run-TryBot: Brad Fitzpatrick <[email protected]>
TryBot-Result: Gobot Gobot <[email protected]>

Author: 0intro

src/syscall/const_plan9.go

@@ -12,6 +12,17 @@ const (
 	O_EXCL    = 0x1000
 )
 
+// Bind flags
+const (
+	MORDER  = 0x0003 // mask for bits defining order of mounting
+	MREPL   = 0x0000 // mount replaces object
+	MBEFORE = 0x0001 // mount goes before others in union directory
+	MAFTER  = 0x0002 // mount goes after others in union directory
+	MCREATE = 0x0004 // permit creation in mounted directory
+	MCACHE  = 0x0010 // cache some data
+	MMASK   = 0x0017 // all bits on
+)
+
 // Rfork flags
 const (
 	RFNAMEG  = 1 << 0