While often similar, these are not the same on Windows. We want to use the page
size on Windows for the pools, but for mmap we need to use the allocation
granularity as the alignment.

On the other platforms these values remain the same.

Use the `giterr_set` function, which actually supports `GITERR_OS`.
The `giterr_set_str` function is exposed for external users and will
not append the operating system's error message.

A non-readable mapping of a file causes an access violation in
the pack tests. Always use PROT_READ to work around this.

Use size_t for page size, instead of long. Check result of sysconf.
Use size_t for page offset so no cast to size_t (second arg to p_mmap).
Use mod instead div/mult pair, so no cast to size_t is necessary.

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.

Needs AmigaOS.cmake now from CMake package at OS4Depot, or contents below:
--8<--
SET(AMIGA 1)
SET(CMAKE_SHARED_LIBRARY_C_FLAGS "-fPIC")
SET(CMAKE_SHARED_LIBRARY_CREATE_C_FLAGS "-shared")
--8<--

On RAM: the .idx and .pack files become links to a .lock and the original download respectively.
Assume some feature (such as record locking) supported by SFS but not JXFS or RAM: is required.