Install fallback drop policies to avoid transmitting unencrypted packets.

During the update of a CHILD_SA (e.g. caused by MOBIKE) the old policy
is first uninstalled and then the new one is installed.  In the short
time in between, where no policy is available in the kernel, unencrypted
packets could have been transmitted.

src/libhydra/kernel/kernel_ipsec.h

@@ -98,6 +98,8 @@ enum policy_priority_t {
 	POLICY_PRIORITY_DEFAULT,
 	/** Priority for trap policies */
 	POLICY_PRIORITY_ROUTED,
+	/** Priority for fallback drop policies */
+	POLICY_PRIORITY_FALLBACK,
 };
 
 /**