Changes

Add "tcp_send_timeout" option to set a TCP send data timeout

When using Stubby as a system DNS over TLS resolver with a Internet
connection that disconnects and reconnects from time to time there is often
a long waiting time (~20 minutes) after the connection reconnects before
DNS queries start to work again.

This is because in this particular case all the upstream TLS TCP
connections in Stubby are stuck waiting for upstream server response.
Which will never arrive since the host external IP address might have
changed and / or NAT router connection tracking entries for these TCP
connections might have been removed when the Internet connection
reconnected.

By default Linux tries to retransmit data on a TCP connection 15 times
before finally terminating it.
This takes 16 - 20 minutes, which is obviously a very long time to wait for
system DNS resolving to work again.
This is a real problem on weak mobile connections.

Thankfully, there is a "TCP_USER_TIMEOUT" per-socket option that allows
explicitly setting how long the network stack will wait in such cases.

Let's add a matching "tcp_send_timeout" option to getdns that allows
setting this option on outgoing TCP sockets.
For backward compatibility the code won't try to set it by default.

With this option set to, for example, 15 seconds Stubby recovers pretty
much instantly in such cases.

Signed-off-by: Maciej S. Szmigiero <mail@maciej.szmigiero.name>
 (commit: 606a88f)

fix compilation without deprecated OpenSSL APIs

Several cmake header checks were missing and added.

Added rsa.h include.

Remove ENGINE_load_dynamic. ENGINE_load_builtin_engines already does
this.

Signed-off-by: Rosen Penev <rosenp@gmail.com>
 (commit: 80cdfb3)

Add platform functions for reporting file errors.

Windows socket error numbers are not reported using errno, but with
WSAGetLastError(). _getdns_errnostr() and friends as implemented on
Windows don't work for errors resulting from file open/close/read/write
etc.

So add a parallel set of functions specifically for file errors.
 (commit: 6439b04)

Windowqs mkstemp: open() needs more permissions.

Specifically:

1. The open must be for read only or read/write - use read/write.
2. The file permission must permit read and write.
 (commit: 42e4cfc)

Add missing rr types: SVCB and HTTPS
 (commit: fa282a5)

Add missing rr types: SVCB and HTTPS
 (commit: 7f606ea)

Fix wrong extension names from merge
 (commit: 55be327)

Update components from Unbound
 (commit: 1184f2b)

Eliminate compiler warnings in tests
 (commit: df2997d)

ChangeLog update for issue #480
 (commit: 7b388a1)

Honour the claim from documentation: When not set (the default), the system default is left alone.
 (commit: 624f688)

Changelog entry for getdns_context_set_tcp_send_timeout() contribution

Thanks a lot @maciejsszmigiero , this looks really valueable!
 (commit: 45ef080)