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_test.go

@@ -0,0 +1,12 @@
+// Copyright (c) Tailscale Inc & AUTHORS
+// SPDX-License-Identifier: BSD-3-Clause
+
+package permissions
+
+import "testing"
+
+func TestPermissionsImpl(t *testing.T) {
+	if err := permissionsImpl(t.Logf); err != nil {
+		t.Error(err)
+	}
+}