(PR14488 partial fix) Make memory reading more robust.

(PR14488 partial fix) Make memory reading more robust.

* runtime/dyninst/runtime.h (assert_is_myproc): New dummy macro.
  (stp_dummy_init): Opens the /proc/self/mem file.
  (stp_dummy_exit): Close the file.
* runtime/dyninst/linux_defs.h (user_mode): New dummy macro.
  (__copy_from_user): Rewrote to read memory from /proc/self/mem, to be
  able to catch errors.
* runtime/stp_string.h (__stp_get_user): Always use __get_user(), no
  matter what arch we're on in dyninst mode.
* tapset/uconversions.stp: Moved from tapset/linux/uconversions.stp, since
  the dyninst runtime can use the functions also.