offlineimap's module: change UI to syslog

The 'syslog' UI "allows better integration with systemd":
http://www.offlineimap.org/doc/Changelog.html#offlineimap-v660-rc2-2015-10-15

nixos/modules/services/networking/offlineimap.nix

···
       description = "Offlineimap: a software to dispose your mailbox(es) as a local Maildir(s)";
       serviceConfig = {
         Type      = "oneshot";
-        ExecStart = "${cfg.package}/bin/offlineimap -u basic -o -1";
+        ExecStart = "${cfg.package}/bin/offlineimap -u syslog -o -1";
         TimeoutStartSec = cfg.timeoutStartSec;
       };
       path = cfg.path;