doctor/permissions: add new check to print process permissions

Since users can run tailscaled in a variety of ways (root, non-root,
non-root with process capabilities on Linux), this check will print the
current process permissions to the log to aid in debugging.

Signed-off-by: Andrew Dunham <andrew@du.nham.ca>
Change-Id: Ida93a206123f98271a0c664775d0baba98b330c7

doctor/permissions/permissions_other.go

@@ -0,0 +1,17 @@
+// Copyright (c) Tailscale Inc & AUTHORS
+// SPDX-License-Identifier: BSD-3-Clause
+
+//go:build !(linux || darwin || freebsd || openbsd)
+
+package permissions
+
+import (
+	"runtime"
+
+	"tailscale.com/types/logger"
+)
+
+func permissionsImpl(logf logger.Logf) error {
+	logf("unsupported on %s/%s", runtime.GOOS, runtime.GOARCH)
+	return nil
+}