Add autodesc and autoexclude options.

This patch implements two optional options for multidir repos: autodesc and
autoexclude.

If autodesc is enabled, the description will be taken from the file named
"_darcs/third_party/darcsweb/desc", inside the repository. If it doesn't
exist, the default will be used.

If autoexclude is enabled, only repositories with a directory named
"_darcs/third_party/darcsweb/" will be listed.

The path "_darcs/third_party/darcsweb/" for darcsweb-exclusive preferences was
chosen following a suggestion by David Roundy in the mail with Subject
"[darcs-users] darcsweb 0.12 and some questions", Message-ID
20051109123238.GB17288@abridgegame.org, date "9 Nov 2005 07:32:44".