Make top level /var/lib/postfix as root:root 0755
After generating custom configs in /var/lib/postfix/conf,
`postfix set-permissions` called, to perform all required tricks
related to queue handling (postfix use file mode bits to keep
some internal statuses, so `chmod -R` not recommended by authors,
see comments in $out/libexec/postfix/post-install for details)
Also post-install script was patched, to skip permission check/update
for files inside $out, as well as symlinks following to $NIX_STORE.
Config file `main.cf` extended with all default directory locations,
to prevent post-install script from guessing and overwrite them.
And finally all actions in activation script snippets performed
by postmap/postalias/postfix tools from current build, not random one
from paths.
