Detect and use %zu for size_t when available

Detect and use %zu for size_t when available

Commit db949b7fade69d7eb1e38ad85d5b822c443306cb was incorrect and
generated warnings on many platforms; we cannot determine the correct
format string for a size_t at compile-time. Instead use the %z length
modifier when we can detect that it is safe to use. Otherwise, fall back
to just %lu on Unix, since it is reasonable to assume
sizeof(size_t) == sizeof(unsigned long) on Unix.

Reviewed-on: http://gerrit.openafs.org/647
Reviewed-by: Russ Allbery <rra@stanford.edu>
Reviewed-by: Simon Wilkinson <sxw@inf.ed.ac.uk>
Reviewed-by: Derrick Brashear <shadow@dementia.org>
Tested-by: Derrick Brashear <shadow@dementia.org>