add basic webserver that can do most of what i need

- serve static files, serving index.html or optionally listings for directories
- redirects
- reverse-proxy, forwarding requests to a backend

these are configurable through the config file. a domain and path regexp have to
be configured. path prefixes can be stripped.  configured domains are added to
the autotls allowlist, so acme automatically fetches certificates for them.

all webserver requests now have (access) logging, metrics, rate limiting.
on http errors, the error message prints an encrypted cid for relating with log files.

this also adds a new mechanism for example config files.

autotls/autotls_test.go

@@ -28,7 +28,7 @@ func TestAutotls(t *testing.T) {
 	if err := m.HostPolicy(context.Background(), "mox.example"); err == nil || !errors.Is(err, errHostNotAllowed) {
 		t.Fatalf("hostpolicy, got err %v, expected errHostNotAllowed", err)
 	}
-	m.AllowHostname(dns.Domain{ASCII: "mox.example"})
+	m.SetAllowedHostnames(map[dns.Domain]struct{}{{ASCII: "mox.example"}: {}})
 	l = m.Hostnames()
 	if !reflect.DeepEqual(l, []dns.Domain{{ASCII: "mox.example"}}) {
 		t.Fatalf("hostnames, got %v, expected single mox.example", l)
@@ -79,7 +79,7 @@ func TestAutotls(t *testing.T) {
 		t.Fatalf("private key changed after reload")
 	}
 	m.shutdown = make(chan struct{})
-	m.AllowHostname(dns.Domain{ASCII: "mox.example"})
+	m.SetAllowedHostnames(map[dns.Domain]struct{}{{ASCII: "mox.example"}: {}})
 	if err := m.HostPolicy(context.Background(), "mox.example"); err != nil {
 		t.Fatalf("hostpolicy, got err %v, expected no error", err)
 	}