feat: add optional variadic args to all commands (#8)

Allow callers to pass additional systemctl flags (e.g. --no-block,
--force) via variadic string args on every exported function.

This is backward-compatible: existing callers without extra args
continue to work unchanged.

Introduces a prepareArgs helper to centralize argument construction,
replacing the duplicated args/UserMode pattern across all functions.

Closes #2

util_test.go

@@ -0,0 +1,62 @@
+package systemctl
+
+import (
+	"reflect"
+	"testing"
+)
+
+func TestPrepareArgs(t *testing.T) {
+	tests := []struct {
+		name     string
+		base     string
+		opts     Options
+		extra    []string
+		expected []string
+	}{
+		{
+			name:     "system mode no extra",
+			base:     "start",
+			opts:     Options{},
+			extra:    nil,
+			expected: []string{"start", "--system"},
+		},
+		{
+			name:     "user mode no extra",
+			base:     "start",
+			opts:     Options{UserMode: true},
+			extra:    nil,
+			expected: []string{"start", "--user"},
+		},
+		{
+			name:     "system mode with unit",
+			base:     "start",
+			opts:     Options{},
+			extra:    []string{"nginx.service"},
+			expected: []string{"start", "--system", "nginx.service"},
+		},
+		{
+			name:     "user mode with unit and extra args",
+			base:     "restart",
+			opts:     Options{UserMode: true},
+			extra:    []string{"foo.service", "--no-block"},
+			expected: []string{"restart", "--user", "foo.service", "--no-block"},
+		},
+		{
+			name:     "daemon-reload no extra",
+			base:     "daemon-reload",
+			opts:     Options{},
+			extra:    nil,
+			expected: []string{"daemon-reload", "--system"},
+		},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			got := prepareArgs(tt.base, tt.opts, tt.extra...)
+			if !reflect.DeepEqual(got, tt.expected) {
+				t.Errorf("prepareArgs(%q, %+v, %v) = %v, want %v",
+					tt.base, tt.opts, tt.extra, got, tt.expected)
+			}
+		})
+	}
+}