Support DTLS

[SeanTAllen]

The library exports DTLS version primitives (DTLS1Version, DTLS1u2Version, DTLSMinVersion, DTLSMaxVersion) and the docstrings on set_min_proto_version and set_max_proto_version list them as supported versions. But SSLContext creates every context with TLS_method(), and OpenSSL rejects DTLS version numbers on a TLS-method context — SSL_CTX_ctrl returns 0 and the setter raises.

Supporting DTLS would mean creating contexts with DTLS_method() when DTLS versions are requested. The version-range validation added in #142 also uses a numeric comparison that gives wrong results for DTLS, since DTLS version numbers decrease with protocol version (DTLS 1.0 = 0xFEFF, DTLS 1.2 = 0xFEFD).

[SeanTAllen]

Going with entirely separate types for DTLS. DTLSContext and DTLS alongside SSLContext and SSL, not shared.

You're not going to use DTLS from TCP code. Sharing types between the two means every type carries methods and states that only apply to one protocol — retransmission timers that are no-ops for TLS, a TCP connection wrapper that doesn't apply to DTLS, runtime checks for which protocol you're actually using. Separate types make the wrong combinations a compile error instead of no-op methods and runtime checks.

Detailed planning starts from here.