Protect against error in user home directory computation

Protect against error in user home directory computation

This is a quick fix. There are comments about other related issues and
better ways to do it.

2013-04-16  Didier Verna  <didier@didierverna.net>

    * src/util.lisp (home-directory): New warning condition.
    (home-directory): Handle TRUENAME errors by returning NIL.
    * src/synopsis.lisp (initialize-instance): Handle NIL user home
    directory in the clon-search-path default value.
    * src/options/path.lisp (convert): Mention the bug about
    HOME-DIRECTORY returning NIL.
    * TODO: Mention the bug about static computation of the default value
    of the clon-search-path option.