util/linuxfw, feature/buildfeatures: add ts_omit_iptables to make IPTables optional

Updates #12614

Change-Id: Ic0eba982aa8468a55c63e1b763345f032a55b4e2
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

ipn/ipn_view.go

@@ -421,6 +421,8 @@ func (v PrefsView) PostureChecking() bool { return v.ж.PostureChecking }
 
 // NetfilterKind specifies what netfilter implementation to use.
 //
+// It can be "iptables", "nftables", or "" to auto-detect.
+//
 // Linux-only.
 func (v PrefsView) NetfilterKind() string { return v.ж.NetfilterKind }
 

ipn/prefs.go

@@ -264,6 +264,8 @@ type Prefs struct {
 
 	// NetfilterKind specifies what netfilter implementation to use.
 	//
+	// It can be "iptables", "nftables", or "" to auto-detect.
+	//
 	// Linux-only.
 	NetfilterKind string