openat: make template easier to use

openat: make template easier to use

* lib/at-func.c (CALL_FUNC): Allow AT_FUNC_USE_F1_COND and
AT_FUNC_F2 to be undefined.
(VALIDATE_FLAG): New macro; use it to reject bad flags.
(AT_FUNC_USE_F1_COND): Change sense to just flag bit.
* lib/fchmodat.c (AT_FUNC_USE_F1_COND): Adjust.
* lib/fchownat.c (AT_FUNC_USE_F1_COND): Likewise.
* lib/openat.c (AT_FUNC_USE_F1_COND) [fstatat, unlinkat]:
Likewise.
* lib/mkdirat.c (AT_FUNC_F2, AT_FUNC_USE_F1_COND): Delete.
* lib/selinux-at.c (AT_FUNC_F2, AT_FUNC_USE_F1_COND)
[getfileconat, lgetfileconat, setfileconat, lsetfileconat]:
Likewise.

Signed-off-by: Eric Blake <ebb9@byu.net>