PR14297: Skip the user's input when scanning for resolved probes

PR14297: Skip the user's input when scanning for resolved probes

* elaborate.cxx (derived_probe::script_location): Sweep the derivation
  chain from its second-to-last, so we get at least one level of
  resolution from the user's input.
* testsuite/systemtap.base/list.exp: Add a case for kernel init_once,
  which demonstrates listing the @file:line is useful to disambiguate.
  Add a case for 'begin' too, the simplest expansion we have.