feat(tcp): socket errors as std::system_error, with the phase that failed

SocketErrorException is now a std::system_error: code() carries the errno
under std::generic_category(), or getaddrinfo's result under the new
ResolverCategory(), whose message() is gai_strerror's text; `phase` says
which step failed (resolve, open, connect, send, receive). It is still a
std::runtime_error with the message ClientTCP always threw, the code's
text after it, so existing handlers keep working.

Before, the exception carried a `kind` and an int, and a caller that had
to print the cause its own way still had to match the message prefix to
learn where the failure happened. The standard type ends both: the
category knows how to print the number, and the phase is a field. A
caller writes "[Errno {}] {}" from code().value() and code().message(),
or compares code() with std::errc::connection_refused.

Breaking: SocketErrorException's `kind` and `code` members are gone;
read code().category() and code().value().

tests/ShouldConnectDualStack/main.cpp

@@ -140,7 +140,7 @@ int main() {
         try {
             ClientTCP client("localhost", closedPort, 2s);
         } catch (const SocketErrorException& e) {
-            refused = e.kind == SocketErrorKind::System && e.code == ECONNREFUSED;
+            refused = e.phase == SocketPhase::Connect && e.code() == std::errc::connection_refused;
         } catch (const std::exception&) {
         }
         Check(refused, "when no address answers, the last failure is thrown, with its errno");