win32: add "zint_dll_vc6" sub-directory with VC6 workspace for

creating "zint.dll" compatible with Windows XP (see ticket #339)
win32: rename README and HOWTO files to uppercase and update
  "win32\README.txt"
general: suppress some VC6 warnings
README.bsd: update

win32/zint_cmdline_vc6/README.txt

@@ -0,0 +1,28 @@
+Harald Oehlmann
+2025-01-27
+
+Why to use VC6 ?
+It avoids DLL Hell as the runtime is present on all Windows Versions since XP.
+I compile on Windows 10 64 bit.
+
+How to compile:
+
+$ZR is the zint root folder (this file is in $ZR\win32\zint_cmdline_vc6)
+
+* get vc6 shell
+if not available in the Start Menu, open a Command Prompt and run
+"C:\Program Files (x86)\Microsoft Visual Studio\VC98\Bin\VCVARS32.BAT"
+
+a) zlib (current version: 1.3.1)
+* put zlib to $ZR\..\zlib
+cd $ZR\..\zlib
+nmake -f win32\Makefile.msc
+-> generates zlib.lib, zlib1.dll
+
+b) lpng (current version: 1.6.46)
+* put libpng to $ZR\..\lpng
+cd $ZR\..\lpng
+nmake -f scripts\makefile.vcwin32
+-> generates $ZR\..\lpng\libpng.lib
+
+c) open zint_cmdline_vc6.dsw in this folder with the msvc6 gui and compile