server implementation is not yet ready

[hannesm]

(while re-reviewing #2, I noticed some shortcomings)

• it has no explicit support for udp, which should be tested and likely requires some more code
• it does not forward received packets elsewhere, but just does the handshake and replies to ICMP
• it should be checked that it properly applies the different timeouts (handshake / ping / ...)
• authentication: it should verify username and password via a user database -- and/or request client certificate and verify against CA
• hardcoded Config.server_generate_connect_options should be generated from the configuration

[hannesm]

and in Engine, the server should validate the configuration (similar to what the client does)

[hannesm]

work done:

• Server: allow any TLS session tickets in established state #211 (TLS tickets)
• decode_tls_data: don't add u_len twice #212
• add a lwt miragevpn_server_notun to ease development #225 (server notun)
• Try to handle --server a bit better #226 (handle --server a bit better)
• more work on server #229 (cipher negotiation, server notun fixes, request_push, tls data when established)
• more work on the server #231 (move is_not_taken around, improved configuration validation)
• Keep TLS state when established #232 (keep tls state - preparation for exitcc)
• bugfix: use latest tls' state for ekm, as reported by @reynir #233
• Fix Use_cc_exit_notify bit value #237
• Server tls-ekm negotiation #235
• Add tls-crypt for our server, require if tls-mode is present as well #242
• Server tls-crypt-v2 support #244
• config: check that server has proto = TCP if specified #246
• Reenable tls-ekm #243
• Miragevpn.server: authentication #249
• server: support client-to-client #252
• Push directive #254
• server unikernel add NAT support #259
• mirage-server: use a block device, fix compilation #260

[hannesm]

overcome by events, implemented now. For some items of the list I opened separate issues.