Tiny helper that builds a *net.Dialer (and tls.Dialer wrapper) with an
optional LocalAddr. Used by the SMTP and IMAP clients so a worker on a
multi-IP box can bind outbound TCP to a specific source IP.
Bind IP comes from an explicit *net.TCPAddr on the client, or falls back
to the WORKER_BIND_IP env var, or finally to the OS default route. The
env-var path is cached once via sync.Once.
4 tests cover explicit bind, nil-passthrough, tls.Dialer composition,
and timeout sanity.
Matthew Meszaros