[Nut-upsuser] NUT on Windows revival

[Jim Klimov]

Hello all,

  After a hectic month in private life, as a byproduct I've got a viable
merger of last released NUT 2.6.5 based Windows-ready codebase (thanks to
the giants active a dozen years ago, on whose shoulders I stood today) and
modern 2.8.x/master, fixing the merge conflicts and build warnings. Some
details were tracked in discussion of
https://github.com/networkupstools/nut/issues/5

  There are caveats, e.g. in a few files code was just hidden by `#ifndef
WIN32` so making it "appear" would be a future trick, and some "silly"
fallbacks for missing methods were hacked to make it build but probably are
not thread-safe etc... but at least binaries do appear and some do run
without crashing on Windows - e.g. usbhid-ups and nut-scanner did see my
test UPS. Fixes originally made for libusb-0.1 were applied to libusb1.c
too, but not tried yet.

  Builds are possible, mostly scripted and documented, see:

* `docs/config-prereq.txt` and `ci_build.sh` for MSYS2 (includes MinGW)
natively on Windows,
* `scripts/Windows/README` and a build script nearby for using mingw
cross-building packages on Ubuntu Linux.

Both of these produce lots of standalone usable binaries installable as a
typical NUT directory tree, which (with copies of DLLs for third-party
projects) "just work".

The adventure starts with:

:; git clone -b Windows-v2.8.0-1 https://github.com/networkupstools/nut nut-win

I really hope that the community members more interested and better versed
in the platform can pick it up from here, to discover and fix the missing
and flawed bits, make sense of the installer, etc. and eventually make this
codebase part of common NUT, multi-platform as it is.

Sone points of interest OTOH:
* c++ and a few drivers are currently effectively "neutered"
* fcntl() code to avoid FD leaks to forked children, is it doable on
Windows?
* loading dlls from one dir, without copies near every exe (see common.c)
* automatically tracking major version number X (from client/Makefile.am)
for LTDL discovery of libupsclient-X.dll without hardcoding it
* localtime_r, gmtime_r use ugly fallbacks where unavoidable
* usb close_dev() crashes
* not all dependencies were tried, probably more build warnings are lurking
to clean up
* installer not investigated
* code mostly looks at "WIN32" macro as a big switch; there should be more
specific configure-script tests for different versions, toolkits, build
envs, IDEs... that are possible. Supported or not, WIN XP may still be a
big thing...
* clean up what `make install` does (e.g. no systemd/SMF on win)
* automate copying third-party dlls for `make install DESTDIR=...`
(recursive ldd?)
* make sure that Linux/Mac/Solaris/BSD/... builds from this branch are not
broken by that big code merge.

On my side, curiosity is satisfied, itch scratched and bogus achievement
achieved, so I plan to rest and then do other things. Really looking
forward to you the reader to make the next iterations.

When this codebase gets "good enough" to at least not fail to build against
more and more third-party projects, I'll look at covering this in NUT CI so
it does not regress with future evolution. This milestone is actually
already viable now, just I'm leaving for vacation :-)
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