Calling cmd.exe in Windows follows a very different pattern from Vim.
The primary difference is that Vim does a nested call to cmd.exe, e.g.
the following call in Vim

    system('echo a 2>&1')

spawns the following processes

    "C:\Program Files (x86)\Vim\vim80\vimrun" -s C:\Windows\system32\cmd.exe /c (echo a 2^>^&1
        ^>C:\Users\dummy\AppData\Local\Temp\VIoC169.tmp 2^>^&1)
    C:\Windows\system32\cmd.exe /c C:\Windows\system32\cmd.exe /c (echo a 2^>^&1
        ^>C:\Users\dummy\AppData\Local\Temp\VIo3C6C.tmp 2^>^&1)
    C:\Windows\system32\cmd.exe  /c (echo a 2>&1
        >C:\Users\dummy\AppData\Local\Temp\VIo3C6C.tmp 2>&1)

The escaping with ^ is needed because cmd.exe calls itself and needs to
preserve the special metacharacters for the last call. However in nvim
no nested call is made, system('') spawns a single cmd.exe process.
Setting shellxescape to "" disables escaping with ^.

The previous default for shellxquote=( wrapped any command in
parenthesis, in Vim this is more meaningful due to the use of tempfiles
to store the output and redirection (also see &shellquote). There is
a slight benefit in having the default be empty because some expressions
that run in console will not run within parens e.g. due to unbalanced
double quotes

    system('echo "a b')

Disable CommandLineToArgvW-standard quoting for cmd.exe.

libuv assumes spawned processes follow the convention expected by
CommandLineToArgvW(). But cmd.exe is non-conformant, so for cmd.exe:
- With system([]), the caller has full control (and responsibility) to
  quote arguments correctly.
- With system(''), shell* options are used.

libuv quoting is disabled if argv[0] is:
- cmd.exe
- cmd
- $COMSPEC resolving to a path with filename cmd.exe

Closes #6329
References #6387

Hope this will make people using feed_command less likely: this hides bugs.
Already found at least two:

1. msgpackparse() will show internal error: hash_add() in case of duplicate
   keys, though it will still work correctly. Currently silenced.
2. ttimeoutlen was spelled incorrectly, resulting in option not being set when
   expected. Test was still functioning somehow though. Currently fixed.

Ref #4615
Ref vim/vim#768