nixos/modules: fix systemd start rate-limits

These were broken since 2016:
https://github.com/systemd/systemd/commit/f0367da7d1a61ad698a55d17b5c28ddce0dc265a
since StartLimitIntervalSec got moved into [Unit] from [Service].
StartLimitBurst has also been moved accordingly, so let's fix that one
too.

NixOS systems have been producing logs such as:
/nix/store/wf98r55aszi1bkmln1lvdbp7znsfr70i-unit-caddy.service/caddy.service:31:
Unknown key name 'StartLimitIntervalSec' in section 'Service', ignoring.

I have also removed some unnecessary duplication in units disabling
rate limiting since setting either interval or burst to zero disables it
(https://github.com/systemd/systemd/blob/ad16158c10dfc3258831a9ff2f1a988214f51653/src/basic/ratelimit.c#L16)

nixos/modules/system/boot/systemd-unit-options.nix

@@ -210,12 +210,21 @@ in rec {
       '';
     };
 
+    startLimitBurst = mkOption {
+       type = types.int;
+       description = ''
+         Configure unit start rate limiting. Units which are started
+         more than startLimitBurst times within an interval time
+         interval are not permitted to start any more.
+       '';
+    };
+
     startLimitIntervalSec = mkOption {
        type = types.int;
        description = ''
          Configure unit start rate limiting. Units which are started
-         more than burst times within an interval time interval are
-         not permitted to start any more.
+         more than startLimitBurst times within an interval time
+         interval are not permitted to start any more.
        '';
     };
 
@@ -245,8 +254,7 @@ in rec {
     serviceConfig = mkOption {
       default = {};
       example =
-        { StartLimitInterval = 10;
-          RestartSec = 5;
+        { RestartSec = 5;
         };
       type = types.addCheck (types.attrsOf unitOption) checkService;
       description = ''

nixos/modules/system/boot/systemd.nix

@@ -243,6 +243,8 @@ let
           OnFailure = toString config.onFailure; }
         // optionalAttrs (options.startLimitIntervalSec.isDefined) {
           StartLimitIntervalSec = toString config.startLimitIntervalSec;
+        } // optionalAttrs (options.startLimitBurst.isDefined) {
+          StartLimitBurst = toString config.startLimitBurst;
         };
     };
   };