Previously down network interfaces without an IPv4 address are
removed from the posix api that lists network interfaces.
That means if you bound a vhost listen socket to a particular
interface, it will fail at startup time.
This patch adds these vhosts to a list, starts the vhost without
a listen socket, and checks to see if the vhost's network interface
has appeared while the rest of lws is running.
If it appears, the listen socket is opened on the network interface
and the vhost becomes reachable.
This adds support for a plugin that can be attached to a vhost
to acquire and maintain its TLS cert automatically.
It works the same with both OpenSSL and mbedTLS backends, but
they can't share auth keys, delete the 'auth.jwk' file as it is
in the example JSON when switching between libs
