textutils: translate_to_ascii() unknown chars fix

* translate_to_ascii() checks the characters one by one in order to
  make sure unrecognized values are ignored ("" instead of
  printing "[?]").  (closes #1754)

* Adds unit test for this case.

Signed-off-by: Federico Poli <federico.poli@cern.ch>
Tested-by: Jan Aage Lavik <jan.age.lavik@cern.ch>
Reviewed-by: Tibor Simko <tibor.simko@cern.ch>