nixos/chrony: Let Chrony control/track RTC drift

- The `prayer` package as well as `services.prayer` have been removed because it's been unmaintained for several years and the author's website has vanished.

- The `chrony` NixOS module now tracks the Real-Time Clock drift from the System Clock with `rtcfile` and automatically adjusts it with `rtcautotrim` when it exceeds the maximum error specified in `services.chrony.autotrimThreshold` (default 30 seconds). If you enabled `rtcsync` in `extraConfig`, you should remove RTC related options from `extraConfig`. If you do not want chrony configured to keep the RTC in check, you can set `services.chrony.enableRTCTrimming = false;`

## Other Notable Changes {#sec-release-23.11-notable-changes}

- A new option `system.switch.enable` was added. By default, this is option is

  stateDir = cfg.directory;

  driftFile = "${stateDir}/chrony.drift";

  keyFile = "${stateDir}/chrony.keys";

  rtcFile = "${stateDir}/chrony.rtc";

  configFile = pkgs.writeText "chrony.conf" ''

    ${concatMapStringsSep "\n" (server: "server " + server + " " + cfg.serverOption + optionalString (cfg.enableNTS) " nts") cfg.servers}

    driftfile ${driftFile}

    keyfile ${keyFile}

    ${optionalString (cfg.enableRTCTrimming) "rtcfile ${rtcFile}"}

    ${optionalString (cfg.enableNTS) "ntsdumpdir ${stateDir}"}

    ${optionalString (cfg.enableRTCTrimming) "rtcautotrim ${builtins.toString cfg.autotrimThreshold}"}

    ${optionalString (!config.time.hardwareClockInLocalTime) "rtconutc"}

    ${cfg.extraConfig}

        '';

      };

      enableRTCTrimming = mkOption {

        type = types.bool;

        default = true;

        description = lib.mdDoc ''

          Enable tracking of the RTC offset to the system clock and automatic trimming.

          See also [](#opt-services.chrony.autotrimThreshold)

          ::: {.note}

          This is not compatible with the `rtcsync` directive, which naively syncs the RTC time every 11 minutes.

          Tracking the RTC drift will allow more precise timekeeping,

          especially on intermittently running devices, where the RTC is very relevant.

          :::

        '';

      };

      autotrimThreshold = mkOption {

        type = types.ints.positive;

        default = 30;

        example = 10;

        description = ''

          Maximum estimated error threshold for the `rtcautotrim` command.

          When reached, the RTC will be trimmed.

          Only used when [](#opt-services.chrony.enableRTCTrimming) is enabled.

        '';

      };

      enableNTS = mkOption {

        type = types.bool;

        default = false;

  };

  config = mkIf cfg.enable {

    meta.maintainers = with lib.maintainers; [ thoughtpolice ];

    meta.maintainers = with lib.maintainers; [ thoughtpolice vifino ];

    environment.systemPackages = [ chronyPkg ];

    services.timesyncd.enable = mkForce false;

    # If chrony controls and tracks the RTC, writing it externally causes clock error.

    systemd.services.save-hwclock = lib.mkIf cfg.enableRTCTrimming {

      enable = lib.mkForce false;

    };

    systemd.services.systemd-timedated.environment = { SYSTEMD_TIMEDATED_NTP_SERVICES = "chronyd.service"; };

    systemd.tmpfiles.rules = [

      "d ${stateDir} 0750 chrony chrony - -"

      "f ${driftFile} 0640 chrony chrony - -"

      "f ${keyFile} 0640 chrony chrony - -"

    ] ++ lib.optionals cfg.enableRTCTrimming [

      "f ${rtcFile} 0640 chrony chrony - -"

    ];

    systemd.services.chronyd =