Fix signedness issues in format strings.

Fix signedness issues in format strings.

Compiling with -Wformat-signedness (not enabled so far) causes many
warnings because of format strings which specify an unsigned type but
correspond to an argument of signed type, or vice versa. This commit
fixes all these mismatches.

For "%u", "%d", "%lu", "%ld" we let the format string match the
type of the argument, but for "%x" we need to cast the argument to
a suitable unsigned type.

After this patch the tree compiles cleanly with -Wformat-signedness
given. The warning will be enabled in a subsequent commit.