Added userdir-like functionality (nginx/git-shell)

- nginx server definitions can have 'userdir: True' variable. If this
  variable is enabled, requests for example.org/~username/ will serve
  files from /srv/www/<username>/userdir/public directory;

- 'userdir' script has been added to git-shell-commands. By using that
  script, you can set work tree of specified git repository to users'
  userdir directory (with optional subdirectories);

- other git-shell commands have been modified to support userdir
  functionality;

- currently userdir directories are served on any site that has
  'userdir' variable enabled in it's definition, but host domain name is
  displayed to the user that enables userdir repository;