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.

Merging main libgit2!
Conflicts:
	CMakeLists.txt
	src/unix/map.c

This converts the map validation function into a macro, tweaks
the GITERR_OS system error automatic appending, and adds a
tentative new error access API and some quick unit tests for
both the old and new error APIs.