do_lookup return values and error code need to change.

Code analysis fixes

See merge request !1

* When include C headers extern "C" tells that here comes a C header, no suprises.

* Using range based for avoid useless temporary variables.

* use const ref when iterating over dir entries.
* use type alias bfs:: is more convinient
* replace a last c-style cast by static_cast

* nullptr provides more type safetieness.