PR16861: reset kprobe struct and improve refresh

PR16861: reset kprobe struct and improve refresh

We need to ensure that the stap_dwarf_kprobe struct is completely
zero'ed out after each unregistration so as not to affect future
registrations which will use the same struct.

We also modify the signature of systemtap_module_refresh so that the
name of the module is passed. This allows us to only update the kprobes
related to that module, rather than checking all of them.

Finally, we also set the priority of the module notifier to 0 to
indicate we don't care in which order we are called (i.e. it shouldn't
matter whether we're called before or after the kprobes callback).