feature/linuxdnsfight: move inotify watching of /etc/resolv.conf out to a feature

tsnet apps in particular never use the Linux DNS OSManagers, so they don't need
DBus, etc. I started to pull that all out into separate features so tsnet doesn't
need to bring in DBus, but hit this first.

Here you can see that tsnet (and the k8s-operator) no longer pulls in inotify.

Updates #17206

Change-Id: I7af0f391f60c5e7dbeed7a080346f83262346591
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

feature/buildfeatures/feature_linuxdnsfight_disabled.go

@@ -0,0 +1,13 @@
+// Copyright (c) Tailscale Inc & AUTHORS
+// SPDX-License-Identifier: BSD-3-Clause
+
+// Code generated by gen.go; DO NOT EDIT.
+
+//go:build ts_omit_linuxdnsfight
+
+package buildfeatures
+
+// HasLinuxDNSFight is whether the binary was built with support for modular feature "Linux support for detecting DNS fights (inotify watching of /etc/resolv.conf)".
+// Specifically, it's whether the binary was NOT built with the "ts_omit_linuxdnsfight" build tag.
+// It's a const so it can be used for dead code elimination.
+const HasLinuxDNSFight = false

feature/buildfeatures/feature_linuxdnsfight_enabled.go

@@ -0,0 +1,13 @@
+// Copyright (c) Tailscale Inc & AUTHORS
+// SPDX-License-Identifier: BSD-3-Clause
+
+// Code generated by gen.go; DO NOT EDIT.
+
+//go:build !ts_omit_linuxdnsfight
+
+package buildfeatures
+
+// HasLinuxDNSFight is whether the binary was built with support for modular feature "Linux support for detecting DNS fights (inotify watching of /etc/resolv.conf)".
+// Specifically, it's whether the binary was NOT built with the "ts_omit_linuxdnsfight" build tag.
+// It's a const so it can be used for dead code elimination.
+const HasLinuxDNSFight = true