Pathmapping, C engine: Added two new actions and two conditions
Pathmapping, C engine: Added two new actions and two conditions

- New actions: map_to_value_of_env_var, replace_by_value_of_env_var,
  and new conditions: if_env_var_is_not_empty, if_env_var_is_empty.
- This commit contains implementations is C, a previous commit
  already added support to the Lua engine.
- These are essential for describing rules that can be executed
  by both mapping engines.

ldbox/ldbox,ldbox/ldbox,ldbox/ldbox,h113331pp/scratchbox2,h113331pp/scratchbox2,h113331pp/scratchbox2,OlegGirko/ldbox,ldbox/ldbox,h113331pp/scratchbox2,ldbox/ldbox,ldbox/ldbox,OlegGirko/ldbox,OlegGirko/ldbox,h113331pp/scratchbox2,OlegGirko/ldbox,OlegGirko/ldbox,h113331pp/scratchbox2,OlegGirko/ldbox