Correct and use AFS_SIZET_FMT

Correct and use AFS_SIZET_FMT

On *nix, a size_t needs either %lu or %u to be interpolated in a string,
depending on if we're on an ILP32 arch or not. Define AFS_SIZET_FMT
accordingly, and actually use AFS_SIZET_FMT when we try to print a
size_t.

This removes numerous -Wformat warnings, at least when building on
i386_linux26.

Reviewed-on: http://gerrit.openafs.org/592
Reviewed-by: Matt Benjamin <matt@linuxbox.com>
Tested-by: Jeffrey Altman <jaltman@openafs.org>
Reviewed-by: Derrick Brashear <shadow@dementia.org>
Tested-by: Derrick Brashear <shadow@dementia.org>