Add option for Unix socket's mask/umask

[jo-so]

I assign the users who should connect via Unix socket to the group the conserver process is using. To let them connect I change the process' umask to 002, but this also changes the permission of the log files and the pid file.

It would be helpful to have an option to set the permission bits of the Unix sockets. Maybe someone wants 777 and controls access with the directory permissions.