Allow configuring the unix socket permissions

[bjoe2k4]

Is your feature request related to a problem? Please describe.

By default the socket is created with the permissions 0o755 on my system and postfix won't be able to connect since it runs as a different user. Putting the postfix user in the mta-sts group won't help either.

Describe the solution you'd like

I'd be nice to be able to configure the permissions via the config file.

Describe alternatives you've considered

For now I've added ExecStartPost=/usr/bin/chmod 666 socketfd to the systemd unit file. This works, but you have to configure the socket twice, once in the config, once in the systemd file.

[Snawoot]

Released in v0.7.0