refactor: improve installer code

- Refactor installer code for better readability and maintainability.
- Move `dagu_server.vsh` to `examples/virt/daguserver/dagu_server.vsh`.
- Remove unnecessary `println` statements.
- Improve error handling in `zinit_installer_actions.v`.
- Update `zinit_installer_actions.v` startup type to systemd.
- Refactor several factory functions.

examples/virt/daguserver/dagu_server.vsh

@@ -0,0 +1,58 @@
+#!/usr/bin/env -S v -n -w -gc none  -cc tcc -d use_openssl -enable-globals run
+
+import freeflowuniverse.herolib.installers.sysadmintools.daguserver
+
+// will call the installer underneith
+
+mut dserver := daguserver.new()!
+dserver.install()!
+dserver.restart()!
+
+println('DAGU installed & running')
+
+mut dagucl := dserver.client()!
+
+// name                 string  // The name of the DAG, which is optional. The default name is the name of the file.
+// description          ?string // A brief description of the DAG.
+// tags                 ?string // Free tags that can be used to categorize DAGs, separated by commas.
+// env                  ?map[string]string // Environment variables that can be accessed by the DAG and its steps.
+// restart_wait_sec     ?int // The number of seconds to wait after the DAG process stops before restarting it.
+// hist_retention_days  ?int // The number of days to retain execution history (not for log files).
+// delay_sec            ?int // The interval time in seconds between steps.
+// max_active_runs      ?int // The maximum number of parallel running steps.
+// max_cleanup_time_sec ?int // The maximum time to wait after sending a TERM signal to running steps before killing them.
+
+mut mydag := dagucl.dag_new(
+	nameswhere: 'test11'
+)
+
+// nr                int     @[required]
+// name              string  // The name of the step.
+// description       string  // A brief description of the step.
+// dir               string  // The working directory for the step.
+// command           string  // The command and parameters to execute.
+// stdout            string  // The file to which the standard output is written.
+// output            ?string // The variable to which the result is written.
+// script            ?string // The script to execute.
+// signal_on_stop    ?string // The signal name (e.g., SIGINT) to be sent when the process is stopped.
+// continue_on_error bool
+// depends           string
+// retry_nr       	int   = 3
+// retry_interval 	int = 5
+
+mydag.step_add(
+	script:         'ls /tmp'
+	retry_interval: 1
+	retry_nr:       3
+)!
+
+mydag.step_add(
+	script:         'ls /root'
+	retry_interval: 1
+	retry_nr:       3
+)!
+
+dagresult := dagucl.dag_register(mydag, start: true)!
+println(dagresult)
+
+println('DAGU should have new steps')