Added t3081 - new circular group test.

Now that commands can have normal links from groups, we always need to do the group check when removing a sticky link.
Added t3082 - make sure environment links persist when removing a group.
Don't read all normal links while parsing.

Since we only need to check if a sticky link is also a normal link when the link is being removed, it's silly to always load all of the normal links during parse time. For directories that have commands with lots of inputs, doing so can really slow down parsing.
Add lua documentation from Rendaw.
Ignore unknown options instead of erroring out

It's hard to share configuration if tup errors out when it encounters
and unknown option. Change this to print a warning instead.