fix manual build under texinfo 5

Texinfo 5 is more assertive about its syntax: macros with
non-alphanumerics have never actually been allowed, but we used to be
able to get away with @& to escape an ampersand under @iftex, and
defining @&key macros under @iffnottex.  Nuh-uh, not any more.  (fixes
lp#1189146)

The details of the indexes, particularly in html format, differ slightly
under texinfo 4 and 5 (related to the trickery around hiding package
prefixes for decent alphabetization).  It might be nice to sort this out
Once And For All, eventually.