introduce vhosts

This patch splits out some lws_context members into a new lws_vhost struct.

 - ssl state and options per vhost
 - SSL_CTX for serving and client per vhost
 - protocols[] per vhost
 - extensions[] per vhost

lws_context maintains a linked list of lws_vhosts.

The same lws_context_creation_info struct is used to regulate both the
context creation and to create vhosts: for backward compatibility if you
didn't provide the new LWS_SERVER_OPTION_EXPLICIT_VHOSTS option, then
a default vhost is created at context creation time using the same info
data as the context itself.

If you will have multiple vhosts though, you should give the
LWS_SERVER_OPTION_EXPLICIT_VHOSTS option at context creation time,
create the context first and then the vhosts afterwards using

  lws_create_vhost(contest, &info);

Although there is a lot of housekeeping to implement this change, there
is almost no additional overhead if you don't use multiple vhosts and
very little api impact (no changes to test apps).

Signed-off-by: Andy Green <andy@warmcat.com>