tidy-up: OS names

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

lib/file.c

@@ -223,7 +223,7 @@ static CURLcode file_connect(struct Curl_easy *data, bool *done)
    * A leading slash in an AmigaDOS path denotes the parent
    * directory, and hence we block this as it is relative.
    * Absolute paths start with 'volumename:', so we check for
-   * this first. Failing that, we treat the path as a real unix
+   * this first. Failing that, we treat the path as a real Unix
    * path, but only if the application was compiled with -lunix.
    */
   fd = -1;