Reservation lease time; reservations by MAC address or client ID

- A reservation can have its own lease time (--lease-time, the
  reservation dialog), over the subnet's. Schema migration 6.
- A reservation with a MAC address and a client ID matches either one
  (a dual-boot machine sends two client IDs from one MAC); before, it
  was found by MAC and then refused. A lease of the reserved address by
  the client's other identity counts as its own.
- The Leases page shows a client's client ID; reserving a lease uses
  both, or one with --by / the dialog's choice.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

internal/validate/validate.go

@@ -294,14 +294,21 @@ func (c *checker) reservations(st *model.State, env *Env) {
 				c.add(Error, "reservation_ip_taken", obj, "%s is already reserved for %s", r.IP, o.Label())
 			}
 			byIP[r.IP] = r
-			k := key{s.ID, "mac " + r.MAC}
-			if r.ClientID != "" {
-				k.id = "id " + r.ClientID
+			// Either of a client's MAC address and client ID finds its
+			// reservation, so neither may be another's.
+			for _, k := range []key{{s.ID, "mac " + r.MAC}, {s.ID, "id " + r.ClientID}} {
+				if k.id == "mac " || k.id == "id " {
+					continue
+				}
+				if o := byClient[k]; o != nil && o != r {
+					c.add(Error, "reservation_client_taken", obj, "this client already has a reservation in %s: %s", s.Label(), o.Label())
+					break
+				}
+				byClient[k] = r
 			}
-			if o := byClient[k]; o != nil {
-				c.add(Error, "reservation_client_taken", obj, "this client already has a reservation in %s: %s", s.Label(), o.Label())
-			}
-			byClient[k] = r
+		}
+		if r.LeaseTime != 0 {
+			c.leaseTime(r.LeaseTime, obj)
 		}
 		if r.Hostname != "" && !validHostname(r.Hostname) {
 			c.add(Error, "reservation_hostname", obj, "%q isn't a valid host name (letters, digits and '-', labels up to 63 characters, separated by dots)", r.Hostname)