attach(
servers) ->ok,err
Bind every listener of every server and spawn their accept loops.
Must be called from inside a running LEV loop. For each server instance:
pre-parses its TLS identities once, binds every cfg.listen spec (or the
single cfg.ip/cfg.port pair when listen is absent), and spawns one
detached accept coroutine per listener. Registers no loop-global signal
handlers; that is serve_all's job. Any bind failure closes the
listeners bound so far and fails the whole attach.