fix: do not stop accepting connections on transient errors

[AndreyOsipuk]

mtg can stay alive but stop accepting connections until it is restarted:

1. Proxy.Serve returns on any Accept error, and runProxy starts it as go proxy.Serve(listener) //nolint: errcheck. When a connection flood exhausts file descriptors (EMFILE), the accept loop ends while the process keeps running; process-level monitoring does not notice.
2. utils.Listener.Accept returns an error when socket options cannot be set on one accepted connection (for example, a client that resets right after the TCP handshake), which ends the accept loop because of a single bad client.

Changes:

• temporary accept errors (EMFILE, ENFILE, ENOBUFS, ENOMEM, ECONNABORTED, ECONNRESET, timeouts) are retried with a pause growing from 5ms to 1s, as net/http does, with a warning in the log; shutdown interrupts the pause;
• a connection whose socket options cannot be set is closed and skipped;
• if Serve still returns an error, runProxy shuts down and returns it, so systemd/docker restarts the process.

Tests: acceptWithRetry survives EMFILE, stops on a closed listener and on shutdown during the pause; IsTemporaryAcceptError and the delay cap; utils.Listener skips a connection with a broken socket (fails on current master). go vet, tests and GOOS=windows/darwin go build ./... pass.

The same fix for mtg-multi (which also covers its MultiListener): dolonet#46.

[AndreyOsipuk]

Found on our production nodes after deploying this change: an ordinary systemctl stop/restart exited with code 1, because shutdown closes the listener and the resulting accept error was reported as a failure. Fixed in the last commit: only a Serve that failed before shutdown started is reported (waitAndShutdown, with tests for both cases). Checked live: SIGTERM now exits with 0.