Fix TLS port leak.
The issue is that the TLS port is assigned to ejabberd_http_sup
because it's spawned during ejabberd_http:start_link/2, which is
called inside ejabberd_http_sup's process.
The fix is to move the initialization of tls into a new function
ejabberd_http:init/2, which is called by proc_lib:spawn_link/3. This
binds the port driver to ejabberd_http, not the supervisor, so they
get cleaned up when the handler closes.