[AC 21.5] Support texinfo 5.x

PATCH 21.5 Remember good old texinfo 4.x? The C implementation was thrown away, and a new implementation was written from scratch in perl for texinfo 5.0, the "Backwards compatibility? What's that?" release. Backwards compatibility was improved in texinfo 5.1, but the new texinfo still doesn't like some of our input files. To be fair, the new texinfo is choking on what appear to be actual errors in our texinfo files. This patch fixes the errors in core. A separate patch will be needed for the packages, which have more perplexing problems.