[management,signal] Make the Let's Encrypt challenge listener address configurable (#7706)

* [management,signal] Make the Let's Encrypt challenge listener address configurable

With Let's Encrypt enabled and --port set to something other than 443,
signal and management also opened a separate challenge listener that was
hard-coded to :443. Non-root deployments, such as the UBI images, could
not start that listener.

Add --letsencrypt-listen-address to both. It defaults to :443, so current
behavior is unchanged. An empty value disables the separate listener for
setups that forward public port 443 to --port, where the main TLS
listener already answers TLS-ALPN-01 challenges.

Signal now fails on startup when the challenge listener cannot bind, and
exits non-zero when a server stops unexpectedly instead of exiting 0. A
failure reported before the run loop waited was previously dropped.
Management no longer opens a new :443 listener on shutdown just to close it.

* [management,signal] Keep the challenge listener change additive

Remove the Signal fail-fast changes from this PR. They change the
behavior that existing installations see after an upgrade, so they move
to a separate PR.

If the challenge listener cannot bind, Signal now logs the error and
continues. The main TLS listener still answers TLS-ALPN-01 challenges.
Management keeps its previous behavior and stops with an error.

The check for an empty address moves to the caller, so the function
does not return a nil listener with a nil error. Also add assertion
messages, guard a nil listener in a test cleanup, and add the flag to
the Signal README.

management/cmd/root.go

@@ -29,6 +29,7 @@ var (
 	mgmtMetricsPort             int
 	disableLegacyManagementPort bool
 	mgmtLetsencryptDomain       string
+	mgmtLetsencryptListen       string
 	mgmtSingleAccModeDomain     string
 	certFile                    string
 	certKey                     string
@@ -70,6 +71,7 @@ func init() {
 	mgmtCmd.Flags().StringVar(&mgmtDataDir, "datadir", defaultMgmtDataDir, "server data directory location")
 	mgmtCmd.Flags().StringVar(&nbconfig.MgmtConfigPath, "config", defaultMgmtConfig, "Netbird config file location. Config params specified via command line (e.g. datadir) have a precedence over configuration from this file")
 	mgmtCmd.Flags().StringVar(&mgmtLetsencryptDomain, "letsencrypt-domain", "", "a domain to issue Let's Encrypt certificate for. Enables TLS using Let's Encrypt. Will fetch and renew certificate, and run the server with TLS")
+	mgmtCmd.Flags().StringVar(&mgmtLetsencryptListen, "letsencrypt-listen-address", ":443", "address of the separate Let's Encrypt challenge listener, used when --port is not 443. Set it empty when public port 443 is forwarded to --port, which answers the challenges itself")
 	mgmtCmd.Flags().StringVar(&mgmtSingleAccModeDomain, "single-account-mode-domain", defaultSingleAccModeDomain, "Enables single account mode. This means that all the users will be under the same account grouped by the specified domain. If the installation has more than one account, the property is ineffective. Enabled by default with the default domain "+defaultSingleAccModeDomain)
 	mgmtCmd.Flags().BoolVar(&disableSingleAccMode, "disable-single-account-mode", false, "If set to true, disables single account mode. The --single-account-mode-domain property will be ignored and every new user will have a separate NetBird account.")
 	mgmtCmd.Flags().StringVar(&certFile, "cert-file", "", "Location of your SSL certificate. Can be used when you have an existing certificate and don't want a new certificate be generated automatically. If letsencrypt-domain is specified this property has no effect")