fail on bad helper_path= arg instead of reverting to default helper path
check for helper existence using stat() instead of access()
when use_helper is active, check that secret file exists before prompting user for password
remove nullok option from gauth_chktoken utility. It is redundant now that existence of secret file is checked in module.
implement nullok and forward_pass logic in module. Remove those options from gauth_chktoken helper.
use gauth_chktoken return value to distinguish between wrong code and another error so that the PAM module can decide whether to prompt the user for a new code in TRY_FIRST_PASS mode.
expanded test suite to cover use_helper option