MINOR: rules: add a new function new_act_rule() to allocate act_rules

MINOR: rules: add a new function new_act_rule() to allocate act_rules

Rules are currently allocated using calloc() by their caller, which does
not make it very convenient to pass more information such as the file
name and line number.

This patch introduces new_act_rule() which performs the malloc() and
already takes in argument the ruleset (ACT_F_*), the file name and the
line number. This saves the caller from having to assing ->from, and
will allow to improve the internal storage with more info.