ext/openssl: add a dtls:// stream transport. Open as Draft.

[GianfriAur]

Adds a dtls:// stream transport (with the dtlsv1.2:// alias) to ext/openssl:
DTLS 1.2 over UDP from the stream API, both client and server. It is the datagram
counterpart of tls://, with the same ssl context options where they apply and
the same Openssl\Session object.

// client
$ctx = stream_context_create(['ssl' => ['cafile' => '/path/ca.pem']]);
$c = stream_socket_client('dtls://198.51.100.1:4433', $errno, $errstr, 5,
        STREAM_CLIENT_CONNECT, $ctx);
fwrite($c, $datagram);
$reply = fread($c, 1500);

// server
$ctx = stream_context_create(['ssl' => ['local_cert' => '/path/server.pem']]);
$s = stream_socket_server('dtls://0.0.0.0:4433', $errno, $errstr,
        STREAM_SERVER_BIND | STREAM_SERVER_LISTEN, $ctx);
$peer = stream_socket_accept($s, 5);

Design. No core changes: it only registers the schemes in ext/openssl,
without touching xp_ssl.c or the stream layer. It is a separate file
(xp_dtls.c) because DTLS is UDP while xp_ssl.c is TCP, and to avoid risking a
regression on the TLS path; it still follows xp_ssl.c's patterns and style and
reuses its shared primitives (Openssl\Session). The server accept uses
DTLSv1_listen(): a stateless cookie exchange (RFC 6347 §4.2.1) against
amplification and spoofing, with a constant-time compare. It is the stable API
present in every released OpenSSL and stays valid for 1.2 even when OpenSSL caps it
in favour of the new listener API for 1.3. The server is single-peer (one peer per
bind); multi-peer is future work.

Features (client and server): peer verification (verify_peer,
verify_peer_name, peer_name, cafile/capath, peer_fingerprint), mutual
authentication (local_cert with passphrase), session resumption
(session_data on the client; an external cache via session_new_cb/
session_get_cb/session_remove_cb plus session_id_context/no_ticket/
session_timeout on the server), keying material export (RFC 5705) and Path MTU
(dtls_link_mtu). The session, session_reused, protocol and cipher are exposed
in stream_get_meta_data()['crypto'].

Tests. A .phpt suite under ext/openssl/tests/dtls_* (client against
openssl s_server, PHP server and client against each other, server against
openssl s_client): handshake and round-trip, verification by CA/name/fingerprint,
mutual auth, robustness against bogus datagrams, keying material, MTU, and full
client- and server-side resumption.

Separate follow-ups (outside the scheme-only scope, they touch the core or
OpenSSL's listener API): a multi-peer server, factoring the code
shared between xp_ssl.c and xp_dtls.c into an xp_common, and enable_crypto
on udp://. There is also a small TODO: php_openssl_dtls_parse_ip_address()
duplicates parse_ip_address_ex() from xp_socket.c.

Discussed on internals: https://externals.io/message/131514.

TODO before removing Draft status

• DTLS 1.3 support (planned for this PR, not implemented yet)
• UPGRADING and NEWS
• php.net documentation
• Rebase on master, green suite, coding standards