Set useragent for cronjobs (wget)

#4020 (closed) set a fake useragent for cron requests.

In most cases the default from wget would also be fine, but if we have to make it different then why not do it properly by setting e.g. --user-agent='ISPConfig managed cronjob'

• Stating a browser that's unrealistically old might one day also be blocked.
• Adding a descriptive name can also help when debugging requests.