Some OSs cannot keep their ideas about file content straight when mixing
standard IO with file mapping. As we use mmap for reading from the
packfile, let's make writing to the pack file use mmap.

This adds tests that try canceling an indexer operation from
within the progress callback.

After writing the tests, I wanted to run this under valgrind and
had a number of errors in that situation because mmap wasn't
working.  I added a CMake option to force emulation of mmap and
consolidated the Amiga-specific code into that new place (so we
don't actually need separate Amiga code now, just have to turn on
-DNO_MMAP).

Additionally, I made the indexer code propagate error codes more
reliably than it used to.

There is a serious bug in the previous tree iterator implementation.
If case insensitivity resulted in member elements being equivalent
to one another, and those member elements were trees, then the
children of the colliding elements would be processed in sequence
instead of in a single flattened list.  This meant that the tree
iterator was not truly acting like a case-insensitive list.

This completely reworks the tree iterator to manage lists with
case insensitive equivalence classes and advance through the items
in a unified manner in a single sorted frame.

It is possible that at a future date we might want to update this
to separate the case insensitive and case sensitive tree iterators
so that the case sensitive one could be a minimal amount of code
and the insensitive one would always know what it needed to do
without checking flags.

But there would be so much shared code between the two, that I'm
not sure it that's a win.  For now, this gets what we need.

More tests are needed, though.

OpenBSD's realpath(3) doesn't require the last part of the path to
exist. Override p_realpath in this OS to bring it in line with the
library's assumptions.

The existing p_lstat implementation on win32 is not quite POSIX
compliant when setting errno to ENOTDIR.  This adds an option to
make is be compliant so that code (such as checkout) that cares
to have separate behavior for ENOTDIR can use it portably.

This also contains a couple of other minor cleanups in the
posix_w32.c implementations to avoid unnecessary work.