PR12612: compatibility with old staprun/stapio

PR12612: compatibility with old staprun/stapio

commit 3abb860f changed the staprun<->runtime message format for
the STP_RELOCATE message, without regard for backward compatibility
for older/mismatching stapruns.  That leads to kernel errors and
no kernel probes and such nonsense.  Let's change the runtime to
interoperate both with old staprun and with the current one.

* runtime/transport/symbols.c (_stp_do_relocation): Also accept
  systemtap-1.3 level relocation message, if the incoming message
  size suggests.