config: support $htmlcache_owning_group and $ctags_owning_group

config: support $htmlcache_owning_group and $ctags_owning_group

Allow the owning group for the htmlcache and ctags subdirectories
to be set independently of the owning group for the rest of the
repository.

In a Girocco configuration where gitweb access is provided on a
read-only basis only, it may still be desirable to allow summary
page caching for performance reasons.

In that case the htmlcache subdirectory must be writable by the
$cgi_user even when the rest of the repository is not.

That can now be accomplished via the $htmlcache_owning_group config
item.

Additionally it may sometimes be desirable for that same read-only
gitweb access to be able to add ctags.  This requires the $cgi_user
to have write access to the ctags subdirectory.

That can now be accomplished via the $ctags_owning_group config
item.

Even if $cgi_user is permitted write access to both of the htmlcache
and ctags directories (but none of the rest of the repositories'
areas) and completely trashes them, the objects contained in the
repositories will remain completely intact and unaffected.

Signed-off-by: Kyle J. McKay <mackyle@gmail.com>