Normalize whitespace in the arguments of <indexterm>

Normalize whitespace in the arguments of <indexterm>

Strip leading and trailing whitespace and replace interior whitespace
by a single space.  This avoids problems with the index generator
producing duplicate index entries for terms that differ only in
whitespace.

Commit dca30da3433c40b5f92f1704c496cda052decef9 actually fixed all the
indexterm elements that would cause this problem at the moment, but in
case it sneaks in again, we're set.