nixos/networkd: add missing [DHCPServer] options

`systemd.network.networks.*.dhcpServerConfig` did not accept all of
the options which are valid for networkd's [DHCPServer] section. See
systemd.network(5) of systemd 247 for details.

nixos/modules/system/boot/networkd.nix

@@ -716,10 +716,17 @@ let
           "NTP"
           "EmitSIP"
           "SIP"
+          "EmitPOP3"
+          "POP3"
+          "EmitSMTP"
+          "SMTP"
+          "EmitLPR"
+          "LPR"
           "EmitRouter"
           "EmitTimezone"
           "Timezone"
           "SendOption"
+          "SendVendorOption"
         ])
         (assertInt "PoolOffset")
         (assertMinimum "PoolOffset" 0)
@@ -728,6 +735,9 @@ let
         (assertValueOneOf "EmitDNS" boolValues)
         (assertValueOneOf "EmitNTP" boolValues)
         (assertValueOneOf "EmitSIP" boolValues)
+        (assertValueOneOf "EmitPOP3" boolValues)
+        (assertValueOneOf "EmitSMTP" boolValues)
+        (assertValueOneOf "EmitLPR" boolValues)
         (assertValueOneOf "EmitRouter" boolValues)
         (assertValueOneOf "EmitTimezone" boolValues)
       ];