@jacobvosmaer noted that from user perspective it is not very handy to
have URL-encoded paths for sockets, and this way it is also not
consistent with everything else in GitLab where we use just
unix://path/to/socket instead of unix://path%2Fto%2Fsocket .

Be friends to users and allow them to configure socket path just with paths
with secondary gitlab_socket option, handler for which just encodes the
path and adds http+unix:// prefix, so internal handling is always by URL.

To protect from inconsistent input, configuration handler checks that
only one of gitlab_url or gitlab_socket can be given at a time.

NOTE CGI::escape() is used instead of URL::escape() because the latter
does not escape '/' characters by default:

    http://stackoverflow.com/questions/14989581/ruby-1-9-3-add-unsafe-characters-to-uri-escape