stapdyn: Add some support for removing instrumentation

stapdyn: Add some support for removing instrumentation

We'll need this for signal-handling cleanup, and for detaching from
-x PID target processes.

* stapdyn/mutatee.h (mutatee::snippets): New vector for all the handles
  to code that we've added with Dyninst insertSnippet.
* stapdyn/mutatee.cxx (mutatee::instrument_dynprobe_target): Save the
  insertSnippet handle in snippets.
  (mutatee::remove_instrumentation): New, remove all the snippets.
  (mutatee::unload_stap_dso): New placeholder for unloading our module
  from the target process.  Dyninst can't do this yet...
  (mutatee::~mutatee): Call remove_instrumentation and unload_stap_dso.