nixos/dovecot: added configFile option and default Restart on-failure, PR #5845

Absolute path is required when one has such postfix configuration
where he/she needs to specify the actual (real) path to active dovecot
config.

Without this commit applied, the dovecot is running in such way:
/nix/store/hashAAA-dovecot-ver/sbin/dovecot -F -c /nix/store/hashBBB-dovecot2.conf

and postfix can't be aware of the value of "hashBBB" via services.postfix.extraConfig = '' ... '';
(it can only be aware of "hashAAA" with ${pkgs.dovecot} parameter)

Also enable Restart on-failure.

Edit: set RestartSec to 1s