Libconfig update to version 1.4.9

It's a maintenance release, that fixes some parsing bugs with invalid escape characters.  Not really critical, but I think it's good to have.  It should also lessen compiler warnings.

I have also re-added the upstream AUTHORS file, which we were probably supposed not to remove, and their changelog, since it may be useful for reference.

Second commit rebuilds the scanner/parser from their yacc/lex sources, since we had done manual changes to them. I prefer the machine-generated version (and the changes are much easier to understand if done on the lex/yacc scripts instead.) Scripts required to rebuild those files (as well as the libconfig documentation) are included, instructions are not, since they'd be specific to a certain bison or flex version and OS. There are some quirks with certain bison and flex versions, but I'm sure anyone who can edit a .l or a .y file will be smart enough to be able to rebuild the scanner in future.

I've done some basic testing here, and I didn't notice anything broken, but feel free to test it more in depth. If there are no objections or reports, I'll merge this in two or three days.