...vhosts from vh destroy on zero refcount

https://github.com/warmcat/libwebsockets/issues/2343

This is needed for, eg, libuv

https://github.com/warmcat/libwebsockets/issues/2338

This is useful for memory constrained systems such as esp32
to improve heap usage minfree/watermark.

On esp32 with simultaneous_ssl_restriction = 2 (MQTT+H1),
Without this change, sys heap minfree ranges from 59k to 71k
  in 100 iterations, average 66.5k.
With this change, sys heap minfree ranges from 64k to 71k
  in 100 interations, average 68.7k

lws_tls_restrict_borrow() returns error when tls restriction limit is
reached. However lws_ssl_close() still calls lws_tls_restrict_return()
to decrease simultaneous_ssl. Thus LWS accepts more than allowed ssl
links, making simultaneous_ssl_restriction useless.

Fix it by tracking lws_tls_restrict_borrow() return value and only
calling lws_tls_restrict_return() if lws_tls_restrict_borrow() is
successful.

https://github.com/warmcat/libwebsockets/issues/2332

This causes the blocking dns lookup to treat EAI_NONAME as immediately
fatal, this is usually caused by an assertive NXDOMAIN from the DNS server
or similar.

Not being able to reach the server should continue to retry.

In order to make the problem visible, it reports the situation using
CLIENT_CONNECTION_ERROR, even though it is still inside the outer client
creation call.

A second chunk of ss / sspc handling did not get cleaned up
along with the other patch from a few weeks ago, it wrongly
treats sspc the same as ss.  This can cause the wrong thing
to be zeroed down, 64-bit and 32-bit builds end up with
different victims.

This patch makes it understand the difference and treat them
accordingly, same as the main for_ss handling.

Later gcc is more accepting, but there's at least one gcc8 toolchain that only likes
to hear "fallthru", or it will flag it

If there's no stash, we didn't get as far as having a session, so just fail
quietly

Some servers will 4xx us if they don't see these headers

https://github.com/warmcat/libwebsockets/issues/2325