This adds h2 http support for the client api.
The public client api requires no changes, it will detect by
ALPN if the server can handle http/2, if so, it will use it.
Multiple client connections using the lws api will be mapped on
to the same single http/2 + tls socket using http/2 streams
that are serviced simultaneously where possible.
AG: unlike openssl, mbedtls does not load the system trust store.
So this change will make client tls operations that work OK on openssl fail on
mbedtls unless you provide the correct CA cert.
This allows lws to distinguish between untrusted CAs, hostname
mismatches, expired certificates.
NOTE: LCCSCF_ALLOW_SELFSIGNED actually allows for untrusted CAs, and
will also skip hostname verification. This is somewhat a limitiation of
the current lws verification process.
AG: improve error reporting up to the CLIENT_CONNECTION_ERROR argument
and add a note specific to mbedtls in the test client. Adapt the test
client to note the CA requirement if built with mbedTLS. Adapt the
minimal test clients to have the CAs available and use them if mbedTLS.
