[syslinux] Librarized chain.c Portions

[Miller, Shao]

I have rebased my 'com32lib_disk' branch at git.zytor.com to compensate
for Gert H.'s recent work.  This purpose of this branch is to shift some
things out of 'chain.c' and into library.

Technically, I think Pierre's work in gpllib/ already has taken some of
these things out of 'chain.c' and made them available.  The licensing is
different, though.  Pierre and I spoke about duplication of effort...  I
sort of thought that those portions in gpllib/ that came from
'chain.c32' could be moved into com32/lib/syslinux/ with the licensing
there (since the authors agree) and the remainder could stay in gpllib/.
Pierre seemed ok with this.

- Shao Miller