Use a control socket directory to restrict access

This provides reasonable security even on Solaris. The sysadmin is
responsible for securing the control socket's ancestors from the
grandparent on.

We could add a cryptographic handshake later if desired.

src/control_common.h

@@ -41,6 +41,7 @@ enum request_type {
 /* This greeting is sent by the server on socket open. */
 typedef struct tinc_ctl_greeting_t {
 	int version;
+	pid_t pid;
 } tinc_ctl_greeting_t;
 
 /* A single request or response header. */