app: Start wpa_supplicant only when Ethernet has no DHCP lease

Probe the configured Ethernet interface (default eth0, overridable via
-ethernet-interface) at startup. If no DHCP-assigned IPv4 is present,
start the wpa_supplicant service so the WiFi backend has something to
talk to; otherwise leave it alone and rely on the wired uplink.

internal/config/config.go

@@ -9,17 +9,18 @@ import (
 )
 
 type Config struct {
-	Bind             string
-	WifiInterface    string
-	HotspotInterface string
-	WifiBackend      string
-	StationBackend   string // "arp", "dhcp", or "hostapd"
-	DHCPLeasesPath   string
-	ARPTablePath     string
-	SyslogEnabled    bool
-	SyslogPath       string
-	SyslogFilter     []string
-	SyslogSource     string
+	Bind              string
+	WifiInterface     string
+	HotspotInterface  string
+	EthernetInterface string
+	WifiBackend       string
+	StationBackend    string // "arp", "dhcp", or "hostapd"
+	DHCPLeasesPath    string
+	ARPTablePath      string
+	SyslogEnabled     bool
+	SyslogPath        string
+	SyslogFilter      []string
+	SyslogSource      string
 }
 
 // ConsolidateConfig fills an Options struct by reading configuration from
@@ -29,15 +30,16 @@ type Config struct {
 func ConsolidateConfig() (opts *Config, err error) {
 	// Define defaults options
 	opts = &Config{
-		Bind:             "127.0.0.1:8080",
-		WifiInterface:    "wlan0",
-		HotspotInterface: "wlan1",
-		DHCPLeasesPath:   "/var/lib/dhcp/dhcpd.leases",
-		ARPTablePath:     "/proc/net/arp",
-		SyslogEnabled:    false,
-		SyslogPath:       "/var/log/messages",
-		SyslogFilter:     []string{"daemon.info wpa_supplicant:", "daemon.info iwd:", "daemon.info hostapd:"},
-		SyslogSource:     "iwd",
+		Bind:              "127.0.0.1:8080",
+		WifiInterface:     "wlan0",
+		HotspotInterface:  "wlan1",
+		EthernetInterface: "eth0",
+		DHCPLeasesPath:    "/var/lib/dhcp/dhcpd.leases",
+		ARPTablePath:      "/proc/net/arp",
+		SyslogEnabled:     false,
+		SyslogPath:        "/var/log/messages",
+		SyslogFilter:      []string{"daemon.info wpa_supplicant:", "daemon.info iwd:", "daemon.info hostapd:"},
+		SyslogSource:      "iwd",
 	}
 
 	declareFlags(opts)