Keep IPv6 router announcements working with forwarding on

With net.ipv6.conf.all.forwarding=1, Linux ignores router announcements
unless accept_ra is 2, so a server that gets its IPv6 route by SLAAC
(e.g. a Raspberry Pi at home) lost IPv6 once the route expired.

The sysctl file now also sets accept_ra=2 for the default and for every
network card and the IPv6 default-route interface, except where
accept_ra is 0. "update" rewrites the file, which fixes existing
installs. A new health check warns while the uplink still has
accept_ra=1.

kernel.go

@@ -3,6 +3,8 @@ package main
 import (
 	"log/slog"
 	"net/netip"
+	"os"
+	"strings"
 	"sync"
 	"time"
 )
@@ -38,6 +40,15 @@ type Kernel interface {
 	Close() error
 }
 
+// readSysctl returns the trimmed content of a /proc/sys file, or "".
+func readSysctl(path string) string {
+	b, err := os.ReadFile(path)
+	if err != nil {
+		return ""
+	}
+	return strings.TrimSpace(string(b))
+}
+
 // Reconciler applies the config to the kernel whenever it is triggered and
 // remembers the outcome for the health report.
 type Reconciler struct {