Implement regression test for path optimizing.

testsuite/arduserial/testpathopt:
* Implemented a test for the path optimizing feature of the
  ser_listarduinos procedure.
testsuite/testdef.h:
* Added a constant for execution error in tests TESTERROR
libarduserial/arduserial.c:
* added some explanatory commentaries to the source code
* stringStack *unstack(stringStack *s) now does nothing if s points to NULL
Prepare release 2.2