fix autotag behavior for relative tags

fix autotag behavior for relative tags

A tag like ./foo is searched for relative to the tagging page.
However, if multiple pages use such a tag, the only one sure
to be in common is in the root, so autocreate it there to
avoid scattering redunadant autocreated tags around the tree.

(This is probably not ideal.)

Also renamed the tagpage and taglink functions for clarity.