This is the correct C99 format code for the size_t type when passed
as an argument to the *printf family.  If the platform doesn't
define it, we assume %lu and just cross our fingers that its the
proper setting for a size_t on this system.  On most sane platforms,
"unsigned long" is the underlying type of "size_t".

Signed-off-by: Shawn O. Pearce <spearce@spearce.org>