pam_selinux.c: rewrite using pam_get_data/pam_set_data

pam_selinux.c: rewrite using pam_get_data/pam_set_data

* modules/pam_selinux/pam_selinux.c (security_restorelabel_tty,
security_label_tty): Remove old functions.
(module_data_t): New structure.
(free_module_data, cleanup, get_module_data, get_item,
set_exec_context, set_file_context, compute_exec_context,
compute_tty_context, restore_context, set_context,
create_context): New functions.
(pam_sm_authenticate, pam_sm_setcred, pam_sm_open_session,
pam_sm_close_session): Use them.