IronPython.SQLite not working on macOS/Linux

While testing some PyPI packages on a recent ipy 3.4 I have discovered that the sqlite3 package bundled with IronPython is not working correctly. After looking into the source code and some debugging I've discovered two major issues ans a few smaller ones.

Major issue #1 is that there is only one "platform compatibility" layer: os_win_c.cs, which, according to the embedded docs is Windows-specific. Nevertheless, it is being unconditionally compiled on all platforms. The compilation succeeds, but the code compatibility alternatives are limited to Windows 95, Windows CE, Windows RT, Windows Phone, Silverlight, and Windows NT (maybe I have missed some other Windows variant...). When run on macOS/Linux, the compatibility methods run out of all platform detection checks and return an error. I can't see how it ever was working on macOS/Linus in any serious way. When I hacked the code to treat macOS as WinNT, it went a little further, but still hit another snag.

Major issue #2: the code uses .NET API that is not supported on macOS. Warnings about that have been recently disabled (#1330). This pertains file locking, which is essential functionality, effectively making sqlite3 unable to open any file. An in-memory database should work though.

The warning itself mentions only problems on macOS, so presumably Linux should be OK (after some hacks). I haven't tested it on Linux yet, but if I find no more snags, I will submit the changes to get at least this platform supported. There is also a possibility to use Mono.Unix compatibility layer on macOS to get partial file locking, but I am not sure if this is fully implemented; the Mono documentation is lacking.

On a wider note, the SQLite implementation that IronPython uses looks very old, unmaintained, supporting obsolete platforms, and not supporting the platforms that IronPython and .NET currently target. So I wonder whether instead of spending time trying to get this code to work, it wouldn't be better to replace it with a modern implementation that is actively maintained (like SQLite.raw).

Start by reading the SQLite platform compatibility layer in os_win_c.cs and the file-locking warning referenced by issue #1330. Reproduce the sqlite3 behavior on macOS and Linux, then determine whether the existing implementation can support both platforms or needs replacement; done means file-backed SQLite works on the supported platforms.