tidy-up: OS names

[vszakats]

Use these words and casing more consistently across text, comments and
one curl tool output:
AIX, ALPN, ANSI, BSD, Cygwin, Darwin, FreeBSD, GitHub, HP-UX, Linux,
macOS, MS-DOS, MSYS, MinGW, NTLM, POSIX, Solaris, UNIX, Unix, Unicode,
WINE, WebDAV, Win32, winbind, WinIDN, Windows, Windows CE, Winsock.

Mostly OS names and a few more.

Also a couple of other minor text fixups.

Closes #14360

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• win32 → Win32/Windows
• Win32 → Windows (in tests and few more places)

[vszakats]

I used macOS when talking about macOS 10.7 and later even though 10.7 wasn't called macOS yet. Existing usage wasn't consistent. Feel free to change it. I left 'OS X' as-is in HISTORY.md.

Windows is often referred to as win32 and Win32, which is stricly speaking not the same, but practically almost always is. I left these as-is anyway in almost all cases to avoid messing it up too. Also some internal flags and API use win32.