Validator: simplified internal structure

While it was very clever, the complex data structure of the tag array
introduced in 'Validator: restructured internal data (a0c9bb25)' comes
with a lot of runtime overhead. It reduces the calls to free and
realloc, when a lot of tags are deleted and inserted subsequently, but
burdens each call with a loop over the linked list of free blocks.

This is even more important, as validator must be fast in checking, as
this is done everytime something is inserted into the DOM-tree, but has
not so tight requirements for registering new tags, as this is merely
done at startup time.