Unconsistent command-line parsing in case of UTF-8 quoted arguments

[Jérôme Froissart]

Hello,

By discussing a merge request on another project [1], I think
billziss-gh found a weirdness in the way Cygwin parses the command
line arguments when non-ASCII characters come into play.

EXPECTED BEHAVIOUR:
cygwin should parse the following command line
    binary.exe --non-ascii "charaçtérs" --ascii "nothing-fancy-here"
as
    argv = ["binary.exe",
            "--non-ascii",
            "chara\xXX\xXXt\xXX\xXXrs",
            "--ascii",
            "nothing-fancy-here"]
    // \xXX\xXX being the UTF-8 encoding of the special characters,
but this does not really matter here
before calling main()

ACTUAL BEHAVIOUR:
it parses it as
    argv = ["binary.exe",
            "--non-ascii",
            "\"chara\xXX\xXXt\xXX\xXXrs\"", // mind the unstripped
quotes here...
            "--ascii",
            "nothing-fancy-here" // ...but not here
    ]

It looks that words containing UTF-8 characters are not properly
stripped when they are surrounded by quotes, unlinke ASCII words.

More examples and a better description is available at [1] (thanks to
billziss-gh for his analysis, much more thorough than mine)
For the record, we wrote a work-around in our specific program, but
handling this issue in Cygwin might be a better way to solve it.

[1]: https://github.com/billziss-gh/sshfs-win/pull/208 (Checking for
quotes around non-ascii usernames passed by Windows)

Thanks for your help! In case you didn't have time, please tell me
where to look at, and I might try to fix it myself and send a patch
proposal if that is easy enough (I have never read Cygwin's code yet).
Jérôme