Support SUID mode for admin_pam_readwrite_service and admin_pam_readonly_service

REST-API pam authentication only uses the traditional PAM auth implementation, which runs with the same user account as MaxScale itself. This may fail if the "maxscale"-user does not have access to password-related files e.g. /etc/shadow. The PAM authenticator module does support a SUID wrapper mode (must be toggled), which runs with root-privileges. The wrapper tool works similarly to the equivalent tool in MariaDB Server. A SUID-mode should be implemented also for the REST-API.