Uses a # to mark section titles, and a >> to indicate code sections,
which otherwise had no formatting to distinguish them from the rest of
the text. This helps with the conversion of the puzzles help text to a
format the sgt-puzzles rockbox port can use.

I ran into this as a knock-on effect from having misspelled a section
name identifier (i.e. I had a \k{nonexistent-name} in my document).
Before I fix the document, I should fix the segfault!

I just happened to run across this clearly unfinished paragraph in
build_huffman_tree(), and when I wrote the rest of it, I realised that
there was actually an implicit input constraint which I hadn't
documented, relating the size of the symbol alphabet to the upper
bound on Huffman code length. (Fortunately, Deflate never violates
that constraint, because both of those values are constant in every
Huffman tree it builds.) So I've also added a pair of assertions, one
of which enforces that constraint.

This is a function I should have introduced a lot earlier while
writing the CHM output code, because I ended up with quite a lot of
annoying loops to add zero-padding of various sizes by going round and
round on the one-byte rdaddc().

Also updates libcharset to the latest revision, which updates _its_
.gitignore (and pulls in other previous fixes too).

I've just set up a script that does code signing by a more sensible
method for cross-compiled Windows builds (i.e. still using the same
underlying technology, but not bothering to fire up a whole Windows
delegation environment that won't get used). So now I can use it.

I don't know how I managed to add a non-TLS URL in this new
code-signing command at around the same time that I'd just switched
over all the ones in my other projects. Must have copied and pasted
from an un-updated checkout, I suppose!

This commit updates the libcharset submodule to incorporate the
autotools-ification that I just pushed to that subproject, and builds
on it by replacing Halibut's own makefile system similarly with an
autotools setup.

The new Makefile.am incorporates both of the old Makefile and
doc/Makefile, so a single run of 'make' should now build Halibut
itself and all the formats of its own documentation, which also means
that the automake-generated 'make install' target can do the right
thing in terms of putting an appropriate subset of those documentation
formats in the assorted installation directories.

The old Makefiles are gone, as is release.sh (which is now obsolete
because autotools's 'make dist' doesn't do anything obviously wrong).
The bob build script is comprehensively rewritten, but should still
work - even the clang-based Windows build can use the
autotools-generated makefile system, provided I do the libcharset
build with a manual override of bin_PROGRAMS to prevent it trying to
build the libcharset supporting utilities (which are not completely
Windows-portable).

Thanks to Leah Neukirchen for pointing out that it was left out of
that special-case rule.

Naturally, after calling the previous commit '1.2', someone had to
instantly find a bug that shows up in the simplest possible example
Halibut invocation :-(