stapdyn: Manage mutator lifetime better

stapdyn: Manage mutator lifetime better

We need to have a global record of mutators in order for dyninst
callbacks to work, since they have no context attached.  But rather than
some singleton factory style, just keep a global list of all mutators
we've seen.

* stapdyn/mutator.cxx (g_mutators): New vector for all mutators.
  (g_dynamic_library_callback): Iterate callbacks for all g_mutators.
  (mutator::dynamic_library_callback): Do the individual work.
  (mutator::mutator): Add this to g_mutators.
  (mutator::~mutator): Remove this from g_mutators.
  (mutator::create, mutator::g_mutator): Removed.
* stapdyn/stapdyn.cxx (main): Use mutator new and auto_ptr delete.