Making makeinfo html-happy

Fixes for warnings issued by makeinfo 5.x (when using some of our
macros: the guy is touchy regarding @ifhtml and new lines) that were
preventing the install-html make target to work (for people that want
local html by texinfo as opposed to the (supposedly fancier)
texi2html-generated version we use for the web).
Fix for apparently stolen link to texinfo
Finer control of debugging window popups

It's now possible to control whether we jump to the debug window on
evaluation errors (geiser-debug-jump-to-debug-p) and whether we show it
all (geiser-debug-show-debug-p).
Almost redundant REPL command to interrupt evaluation

Works almost identically to the stock C-c C-c, with just marginally
better output aaand being well behaved when interrupting infinitely
looping functions such as (define (f) (newline) (let loop () (loop))) --
cf. issue #29 on github.

We like it so much that it replaces the old one.
A bit of NEWS
Guile: font lock for all components of module names
Possible fix for scanning problem

Apparently, the nesting level returned by emacs's syntax parser can be
negative (presumably when it gets confused), and we were not avoiding
calling backward-up-list when that happened.

Could or could not address issue #41...