nvidia driver workaround atexit handlers for pam auth

this works around an apparently atexit() handler in nvidia drivers
that if you fork() a parent and do NOT exec() then the child process
will end up messing with the parent processes access to the nvidia
devices/display/whatever i believe via an atexit() handler because
moving from exit() to _exit() solves the issue. fixes T6197

@fix

src/bin/e_auth.c

@@ -236,7 +236,7 @@ e_auth_begin(char *passwd)
    if (pamerr != PAM_SUCCESS)
      {
         free(current_user);
-        exit(1);
+        _exit(1);
      }
    pamerr = pam_authenticate(da.pam.handle, 0);
    pam_end(da.pam.handle, pamerr);
@@ -246,10 +246,10 @@ e_auth_begin(char *passwd)
    if (pamerr == PAM_SUCCESS)
      {
         free(current_user);
-        exit(0);
+        _exit(0);
      }
    free(current_user);
-   exit(-1);
+   _exit(-1);
 
    return 0;
 }