lws_dll2 removes the downsides of lws_dll and adds new features like a
running member count and explicit owner type... it's cleaner and more
robust (eg, nodes know their owner, so they can casually switch between
list owners and remove themselves without the code knowing the owner).
This deprecates lws_dll, but since it's public it allows it to continue
to be built for 4.0 release if you give cmake LWS_WITH_DEPRECATED_LWS_DLL.
All remaining internal users of lws_dll are migrated to lws_dll2.
If you have multiple vhosts with client contexts enabled, under
OpenSSL each one brings in the system cert bundle.
On libwebsockets.org, there are many vhosts and the waste adds up
to about 9MB of heap.
This patch makes a sha256 from the client context configuration, and
if a suitable client context already exists on another vhost, bumps
a refcount and reuses the client context.
In the case client contexts are configured differently, a new one
is created (and is available for reuse as well).
!!! WIP
This implements the "genec" layer wrapping mbedtls + openssl
ECDH support.
API tests are added for the parts that are implemented so far.
Stuff related to ec at all, like keys, are prefixed lws_genec_.
Stuff specific to ECDH are prefixed lws_genecdh_.
