openat: simplify use of at-func.c * lib/at-func.c (includes): Include prerequisites here, to simplify requirements on client files. * lib/openat-priv.h: Add double-inclusion guard. * lib/faccessat.c (includes): Simplify. * lib/fchmodat.c (includes): Likewise. * lib/fchownat.c (includes): Likewise. * lib/mkdirat.c (includes): Likewise. * lib/mkfifoat.c (includes): Likewise. * lib/symlinkat.c (includes): Likewise. Signed-off-by: Eric Blake <ebb9@byu.net>