PR15029: Reflect script errors in stapdyn's exit status

PR15029: Reflect script errors in stapdyn's exit status

This adds a new stapdyn.h function to query the script/module's exit
status.  One wishes session_exit had returned this in the first place...

Tests beginenderror.exp and trycatch.exp were failing on dyninst, and
are now working with this fix.

* runtime/dyninst/stapdyn.h: Declare stp_dyninst_exit_status();
* runtime/dyninst/runtime.h: Implement it from a static global.
* runtime/dyninst/transport.c: Set that global on "ERROR:" messages,
  like stapio does with its error_detected flag.
* stapdyn/mutator.cxx (mutator::exit_status): New, looks for the new
  function in the module to get status.
* stapdyn/stapdyn.cxx (main): Call mutator::exit_status().