In std.process.ArgIteratorWindows, don't treat unclosed quotes like they're escaped

[cartr]

Currently, if std.process.ArgIteratorWindows's argument parser sees a quotation mark that doesn't have a corresponding closing quote, it acts like it's escaped. For example, a b "c d is parsed as four arguments: a, b, "c, d.

This PR removes this behavior, so a b "c d is instead parsed as three arguments: a, b, c d. This better matches the way other Windows commands parse arguments---TYPE "a b.txt prints the contents of a file called a b.txt---and the way CommandLineToArgvW works. This also makes ArgIteratorWindows a little simpler/faster since it no longer needs to count the total number of quotation marks in the command line.

(It's possible the current behavior was introduced because if you pass a string like ab"cd to CommandLineToArgvW, the returned string includes the quotation mark. But I think this is because of a different oddity in CommandLineToArgvW -- see #2222 (comment) -- and it doesn't happen if you pass a string like a b"cd that has a space somewhere before the quotation mark.)

[andrewrk]

Thanks!