docs cleanup patch

Hi all,
Here's a patch which mostly fixes broken URLs in code comments.

Summary of doc. changes:
* heapfuncs.c: fix awkward comment phrasing

I also tried to fix as many broken URLs as I could find.
* imath.h, imath.c: homepage for M.J. Fromberger moved
* sha1.c, sha1.h: new location of FIPS pub 180-1
* sha2.c: changed URL of PDF describing SHA-256/384/512 to one of the
only active links I could find. archive.org still has the old copy at
http://web.archive.org/web/*/http://csrc.nist.gov/cryptval/shs/sha256-384-512.pdf
* spell.h: I think this was just pointing to the manpage for
hunspell. I changed to the project page for hunspell, though maybe
it'd be better to point to some other copy of the man page somewhere,
e.g. http://pwet.fr/man/linux/fichiers_speciaux/hunspell
* dirmod.c: codeproject.com has shuffled pages

Didn't change:
* be-secure.c: the URL http://www.skip-vpn.org/spec/numbers.html is
down, and I can't find an active copy anywhere else, so I didn't try
to change this. archive.org still has it though (
http://web.archive.org/web/20011212141438/http://www.skip-vpn.org/spec/numbers.html
)

Josh