Do some cleanup of the SDT code. In particular,

* Remove the unused sdt cdev.
* Don't bother keeping a list of probes in struct sdt_prov; it's not needed.
* Invoke sdt_load and sdt_unload from the module handler instead of
  registering separate SYSINITs.
* Keep to within 80 columns.
* Check for errors from dtrace_unregister().