* Comment out references to mod_lock. It doesn't make sense to use it on
  FreeBSD, since it has nothing to do with the kernel linker lock.
* Get rid of the SDT lock for now. I don't think it's needed since all
  manipulation of the global SDT provider list is done with the linker
  lock held.

With these (and some other) changes, SDT probe/provider creation and
destruction seems to be working properly.