ikiwiki/ tips/ mathopd permissions

When using mathopd to serve ikiwiki, be careful of your Umask settings in the mathopd.conf.

With `Umask 026` in mathopd.conf, editing pages resulted in the following errors and a 404 page when the wiki tried to take me to the updated page.

    append_indexes: cannot open .../[destdir]/[outputfile].html
    open: Permission denied

With `Umask 022` in mathopd.conf, editing pages works.

Hopefully this prevents someone else from spending ~2 hours figuring out why this wouldn't work.

> More generally, if your web server uses a nonstandard umask or you're getting permissions related problems in the cgi log when using ikiwiki, you can force ikiwiki to use a sane umask via the `umask` setting in ikiwiki's own setup file. --Joey