Add an option to configure permission on dump

[l00ptr]

We add an option / configuration item to define the permissions for the dumps created with pg_back. We reuse / configure the 0600 permissions by default to ensure some compatibility with the old (non configurable) approach.

We probably need to discuss and improve what we should do for the parent directory when using 'd' for the dump format. Currently we still apply the 0700 mode in such situation, but maybe we should define a specific option or reuse the one we add right now.