show: don't use hex literals in JSON output

show: don't use hex literals in JSON output

JSON does not support hex literals (0x..) so numbers must be formatted
as %d instead of %x.

Currently, the possible values for the gmime error code are 1 (expired
signature), 2 (no public key), 4 (expired key) and 8 (revoked key).
The other possible value is 16 (unsupported algorithm) but obviously
it is much more rare. If this happens, the current code will add
'"errors": 10'. This is valid JSON (it looks like a decimal number)
but it is incorrect (should be 16, not 10).

Since this is just an issue in the JSON encoder, no changes are needed
on the Emacs side (or in other UIs using the JSON output).