html output: escaping of & in title

[Ralf Wildenhues]

`makeinfo --html' from Texinfo 4.8 does not escape ampersands in titles,
for example the Autoconf manual has

| * Program & Function Index::    Index of those with portability problems

and it generates:
| title="Program & Function Index"

BTW, this originated here (the thread may have more hints to other
issues):
http://lists.gnu.org/archive/html/autoconf-patches/2006-03/msg00070.html

Cheers, and keep up the good work!
Ralf