Pass a secret value to sudo_intercept.so and verify after policy check.

The goal is to make it harder for someone to have a fake policy checker.
This will not stop a determined adversary since the secret is present
in the address space of the running process.

src/sudo_exec.h

@@ -80,6 +80,7 @@
 #define SESH_ERR_SOME_FILES 33		/* copy error, some files copied */
 
 #define INTERCEPT_FD_MIN    64		/* minimum fd so shell won't close it */
+#define INTERCEPT_REQ_SEC   42		/* request intercept secret */
 #define MESSAGE_SIZE_MAX    2097152	/* 2Mib max intercept message size */
 
 /*