vfs: Make OpenFile and friends return EINVAL if O_RDONLY and O_TRUNC

Before this change Open("name", os.O_RDONLY|os.O_TRUNC) would have
truncated the file.  This is what Linux does, but is counterintuitive.
POSIX states this is undefined, so return an error in this case
instead.  This preserves the invariant O_RDONLY => file is not
changed.

cmd/mount/fs.go

@@ -91,6 +91,8 @@ func translateError(err error) error {
 		return fuse.Errno(syscall.EROFS)
 	case vfs.ENOSYS:
 		return fuse.ENOSYS
+	case vfs.EINVAL:
+		return fuse.Errno(syscall.EINVAL)
 	}
 	return err
 }