Implement PEP 235: Import on Case-Insensitive Platforms.

Implement PEP 235:  Import on Case-Insensitive Platforms.
    http://python.sourceforge.net/peps/pep-0235.html

Renamed check_case to case_ok.  Substantial code rearrangement to get
this stuff in one place in the file.  Innermost loop of find_module()
now much simpler and #ifdef-free, and I want to keep it that way (it's
bad enough that the innermost loop is itself still in an #ifdef!).

Windows semantics tested and are fine.

Jason, Cygwin *should* be fine if and only if what you did before "worked"
for case_ok.

Jack, the semantics on your flavor of Mac have definitely changed (see
the PEP), and need to be tested.  The intent is that your flavor of Mac
now work the same as everything else in the "lower left" box, including
respecting PYTHONCASEOK.

Steven, sorry, you did the most work here so far but you got screwed the
worst.  Happy to work with you on repairing it, but I don't understand
anything about all your Mac variants.  We need to add another branch (or
two, three, ...?) inside case_ok.  But we should not need to change
anything else.