Return PAM_AUTH_ERR when a system user. This prevents the pam module
from returning success without asking for authentication credentials!

diff --git a/src/pam_sshauth.c b/src/pam_sshauth.c
index 939ea79..879c9e5 100644
--- a/src/pam_sshauth.c
+++ b/src/pam_sshauth.c
@@ -101,7 +100,7 @@ pam_sm_authenticate (pam_handle_t * pamh, int flags, int argc,
     }
 
   /*
-   * Is it a system user?  Succeed.
+   * Is it a system user?  Fail.
    */
 
   pam_debug (pamh, "username %s", username);
@@ -110,7 +109,7 @@ pam_sm_authenticate (pam_handle_t * pamh, int flags, int argc,
     {
       if (pwent->pw_uid < UID_MIN)
         {
-          return PAM_SUCCESS;
+          return PAM_AUTH_ERR;
         }
     }