Add option to append mirror configuration instead of overwriting

[phillebaba]

This change adds a new option to append to existing mirror configuration, which is useful when using Spegel together with other mirrors.

The appending will be done to the contents in the backup directory. As files are only backed up the first time Spegel runs it means that the original mirror configuration will be persisted and reconfigured every time Spegel restarts. The feature is disabled by default to keep the original behavior.

Fixes #413

[bittrance]

This looks reasonable. The central code is a little hard to follow, see review comment for suggestion.

[bittrance]

I think I would have formulated this as:

hf := hostFile{
	Server:      server,
	HostConfigs: map[string]hostConfig{},
}
backupUsed := false
if appendMirrors {
	var err
	fp := path.Join(configPath, backupDir, registryURL.Host, "hosts.toml")
	hf, backupUsed, err = loadBackupHostFile(fs, fp)
	if err != nil {
		return err
	}
}

Using backupUsed we can then generate separate log messages for the separate cases rather than generating an extra log message in the append case.

This leads to three improvements (imo):

• it is clear we read backups from the helper name and the parameter can retain the nameappendMirrors instead of switching to appnedToBackup, and
• it is easier to see that we are replacing hf in the append case, and
• denser log output

[phillebaba]

I totally agree with you.

I had the intention of refactoring this code at a later time when implementing the CRIO version. After some trouble understanding my own code I chose to refactor it now. Added your suggestion to reduce the amount of logs.