PR15656: Check allow_execmod for stapdyn attach

We already check for deny_ptrace and allow_execstack before stapdyn can
proceed, but it turns out that allow_execmod is also important for
Dyninst to attach to processes (e.g. stapdyn -x PID).

* stapdyn/dynutil.cxx (check_dyninst_sebools): If we're going to be
  attaching to a process, check allow_execmod too.
* stapdyn/stapdyn.cxx (main): Indicate whether we're attaching.