Add support for special $user repositories

Add support for special $user repositories

Extra syntax in the config file ($user) will now denote wildcard-style
user repositories. This allows us to add support for users to have
their own public repos without having to do tedious admin work for all
of them.

This involves changes to:

 * access.py to add lookups for $user access on top of normal access;
   $user is treated as a wildcard where necessary when doing
   filesystem path lookups, but used for permission checking too: the
   magic user "$user" can now be added to the members list in
   configuration groups.

 * gitweb.py to add code to deal with the $user wildcards when
   generating the description and project list files that gitweb
   uses. Where we can determine the user's credentials locally, use
   full names. Some code refactoring to fit with existing code
   better. Also: don't assume that close and rename are safe: force
   flush and fsync too

 * example.conf to add entries for user directories. Show how the user
   directories can be made to work