serverconfig: find username on windows too, to set dbname

http://camlistore.org/issue/215

Change-Id: I0599b8061d91c2f6a4ceaa00e54bd0b899540c97
Use osutil.Username in more places.

Change-Id: Id1154fc16c98b3c72d425a4a8adae9c6c0bb2c04