portabtest: Code cleanup
portabtest: Only use one exit code to indicate errors
portabtest: Actually test functions

Test functions snprintf(), strlcpy(), strlcat(), vsnprintf() for
correctness, not only existance (which was quite useless, because
if they weren't available, the program could not have been linked
at all ...).
portabtest: Add checks for strdup(), strndup(), and strtok_r()