[tex-eplain] Bug in index proofing macros

[Karl Berry]

    -  \ifindexproofing \insert\@indexproof{\indexproofterm{#1}}\fi
    +  \ifindexproofing \insert\@indexproof{\indexproofterm{#1}}\allowhyphens\fi

This also looks ok to me.  I can't think of any problem it will cause
offhand, although inserting penalties and glue is always tricky
business.  Dorai, Stepan?

Two minor things:

1) there should be a comment above here explaining that otherwise
the "infinitesmal" is not hyphenated in \sidx{Infinitesimal}infinitesimal.

2) there's no need for \@zzz.  \z at skip (from plain) can be used.

Thanks Oleg!

karl