docs: detect bad links in all docs, not just some...

I don't understand the behavior of the w3c's checklink tool.  Their
manpage is ambiguous, and the observed behavior does not match either
interpretation i can come up with.

The observed behavior is that when multiple input files are given on
the command line, the first file is checked correctly but bad links in
later files are sometimes missed.

This commit runs checklink on each html file individually, instead of
running checklink once on all the html files together.  This finds more
broken links than before...

Since this commit identifies additional problems with our docs, I changed
the Submakefile to run checkref with "warn on failure" mode.