Support client default routes for Linux (#1667)

All routes are now installed in a custom netbird routing table.
Management and wireguard traffic is now marked with a custom fwmark.
When the mark is present the traffic is routed via the main routing table, bypassing the VPN.
When the mark is absent the traffic is routed via the netbird routing table, if:
- there's no match in the main routing table
- it would match the default route in the routing table

IPv6 traffic is blocked when a default route IPv4 route is configured to avoid leakage.

util/net/dialer_generic.go

@@ -0,0 +1,19 @@
+//go:build !linux || android
+
+package net
+
+import (
+	"net"
+)
+
+func NewDialer() *net.Dialer {
+	return &net.Dialer{}
+}
+
+func DialUDP(network string, laddr, raddr *net.UDPAddr) (*net.UDPConn, error) {
+	return net.DialUDP(network, laddr, raddr)
+}
+
+func DialTCP(network string, laddr, raddr *net.TCPAddr) (*net.TCPConn, error) {
+	return net.DialTCP(network, laddr, raddr)
+}