stapdyn: Fork output from stdout/stderr

stapdyn: Fork output from stdout/stderr

We're still using the target's stdio (PR14491), but we're now using
separate FILE handles to do it, so we're not affected by the target
closing its own stdout early.

* runtime/dyninst/io.c (_stp_out, _stp_err): Private FILE handles.
  (_stp_clone_file): Clone a FILE handle, also setting FD_CLOEXEC.
  (_stp_warn, _stp_error, _stp_softerror, _stp_dbug): Use _stp_err.
* runtime/dyninst/print.c )_stp_print_flush): Use _stp_err and _stp_out.
* runtime/dyninst/runtime.h (stp_dyninst_ctor): Clone stderr and stdout.
  (stp_dyninst_dtor): Close _stp_err and _stp_out.