* taptun: add support for Windows
Looking for feedback only at this point, because it's the first time I've attempted any OSD stuff - be gentle.
This extends the existing taptun OSD module to support Windows through the TAP-Windows6 driver (https://github.com/OpenVPN/tap-windows6).
* TAP-Windows6 is GPLv2, however only the header file is required in MAME, and the driver itself is entirely optional.
* I've tried to minimise the size of the diff, rather than completely separate the Windows/non-Windows implementations - not sure which is preferable in this case.
* The license file has Mac(?) line-endings - unsure if they should be normalized or used verbatim.
* Uncertain about the non-Windows case (or the Windows SDL case) - existing code uses __linux__, but I'm not clear on how that works with OSX, for example?
* I can't claim this actually "works" yet, because I don't know enough to configure the tap end of it properly to get networking doing something useful, but I will get there and correct anything necessary in a subsequent real PR.
* minor fixes/improvements (nw)
* Use Unicode Windows APIs, and UTF-8 for MAME
* Deal with unterminated registry string values
* Cancel any pending I/O in destructor
* softfloat3: fix msvc build (nw)
Is this how we do these things? I assume we don't touch the library code itself, and have to "fix" any problems via this header instead?
* softfloat3: do we like this better? (nw)
* softfloat3: it gets the hose again (nw)
* softfloat3: sonofa (nw)
This is a pre-usage import so everyone can get things in line for various compilers. GCC/Clang should be OK on most targets as-is;
MSVC may need to edit 3rdparty/softfloat3/build/MAME/build.h to conform. Please report failure/success with various compilers.
(nw) In future, could people please use stable branches when updating
3rd-party libraries, not unstable development snapshots? It will help
with stability.
