MINOR: stream: add "last_rule_file" and "last_rule_line" samples

MINOR: stream: add "last_rule_file" and "last_rule_line" samples

These two sample fetch methods report respectively the file name and the
line number where was located the last rule that was final. This is aimed
at being used on log-format lines to help admins figure what rule in the
configuration gave a final verdict, and help understand the condition
that led to the action.

For example, it's now possible to log the last matched rule by adding
this to the log-format:

  ... lr=%[last_rule_file]:%[last_rule_line]

A regtest is provided to test various combinations of final rules, some
even on top of each other from different rulesets.