helper/bwrap: implement sync fd

This is required by wayland security-context-v1.

Signed-off-by: Ophestra Umiker <cat@ophivana.moe>

helper/bwrap/config.set.go

@@ -136,3 +136,10 @@ func (c *Config) SetGID(gid int) *Config {
 	}
 	return c
 }
+
+// SetSync sets the sync pipe kept open while sandbox is running
+// (--sync-fd FD)
+func (c *Config) SetSync(s *os.File) *Config {
+	c.sync = s
+	return c
+}