Convert SGML IDs to lower case

IDs in SGML are case insensitive, and we have accumulated a mix of upper
and lower case IDs, including different variants of the same ID.  In
XML, these will be case sensitive, so we need to fix up those
differences.  Going to all lower case seems most straightforward, and
the current build process already makes all anchors and lower case
anyway during the SGML->XML conversion, so this doesn't create any
difference in the output right now.  A future XML-only build process
would, however, maintain any mixed case ID spellings in the output, so
that is another reason to clean this up beforehand.

Author: Alexander Lakhin <exclusion@gmail.com>