Fix remaining pointer-signedness warnings for base64.

* sexp-transport-format.c (base64_encode_in_place): New helper
function.
(sexp_transport_vformat): Use it.

* testsuite/base64-test.c (test_fuzz_once): Update to use char
type where appropriate.
(test_main): Use helper functions base64_encode_in_place and
base64_decode_in_place (copied to this file).

* tools/pkcs1-conv.c (base64_decode_in_place): New helper
function.
(decode_base64): Use it.