Add CVE-2001-1279 (print-rx.c stack overflow) vulnerability to tcpdump.

This is stack based buffer overflow in the print-rx.c code that can be
used to execute arbitrary code on the target system.  For the purpose
of this demo a simple return-oriented programming "gadget" function
has been added to tcpdump.c. -DCHERI_TCPDUMP_VULNERABILITY must be
used to active the vulnerability code.