PAM has a pam_start_confdir() which allows to define the configuration
directory where all services are located.
This is useful to define your own service on tests in particular, so
that you can control your stack and be independant of the host when
running them.
Allow defining this configuration directory, with a new StartConfDir
function.
Also, allow pre-checking for the API availability with
CheckPamHasStartConfdir().
