pbuild should auto-retry downloads

[bmwiedemann]

When pbuild tries to download its BuildRequires and the download fails, it does a hard stop.
However I would prefer it to retry later, so that slow/long unattended builds are more reliable.

This could be a command-line option, e.g --retries -1 to retry forever, --retries 0 for the current behaviour and --retries 2 to stop after the third try... It could use some exponential backoff or just retry every minute.