- add curl_int64_t signed 64-bit type for lib use
- define CURL_PRId64, CURL_PRIu64 format ids
- use curl_int64_t in vquic
curl_int64_t signed complements the existing curl_uint64_t unsigned.
Note that `curl_int64_t` and `int64_t` are assignable from each other
but not identical. Some platforms with 64 long type defint int64_t as
"long long" (staring at macOS) which messes up things like pointers and
format identifiers.
Closes https://github.com/curl/curl/pull/13293
9287563e86