openat: Increase OPENAT_BUFFER_SIZE from 512 to at least 1024

openat: Increase OPENAT_BUFFER_SIZE from 512 to at least 1024

This avoids heap allocation for file names whose lengths are in
the range 512..1023, with the upper bound increasing to at most
4031 depending on the platform's PATH_MAX.  (We do not want
pathmax.h here as it might supply a non-constant PATH_MAX.)
* lib/openat-priv.h (SAFER_ALLOCA_MAX, SAFER_ALLOCA): New macros.
Perhaps they should be moved to malloca.h?
(OPENAT_BUFFER_SIZE): Use them.