Quotation in System.Process.system for Windows

Moved from https://ghc.haskell.org/trac/ghc/ticket/5376:

Wagnerdm writes: I had a bit of fun recently tracking down quoting issues with the "system" command in Windows. For the examples below, I'll consistently use "Windows> " as the beginning of some text sent to the Windows command prompt cmd.exe, and use "GHC> " as the beginning of some text sent to a ghci session running in cmd.exe with System.Cmd imported.

The situation is this: I want to hand off a command line which has both a possibly-quoted command name and a (definitely) quoted argument. For concreteness, let's use "more" as the command and "foo.txt" as the argument, so that you can follow along at home on your favorite Windows system.

```
Windows> echo foo > foo.txt
Windows> "more" "foo.txt"
foo
```

All good so far. But:

```
GHC> system "\"more\" \"foo.txt\""
'more" "foo.txt' is not recognized as an internal or external command, operable program or batch file.
ExitFailure 1
```

After some digging, I discovered that system is shipping out to cmd /c, and indeed:

```
Windows> cmd /c "more" "foo.txt"
'more" "foo.txt' is not recognized as an internal or external command, operable program or batch file.
```

I don't know what the _right_ fix is. However, after a bit of playing around, I discovered the following:

```
Windows> cmd /c ""more" "foo.txt""
foo
GHC> system "\"\"more\" \"foo.txt\"\""
foo
ExitSuccess
```

Wrapping commands with an extra pair of double-quotes this way seemed to give behavior matching the bare cmd.exe for all the examples I could think of, even ones I thought it would break. For example:

```
GHC> system "\"more foo.txt\""
foo
ExitSuccess
```

If this turns out to be the right thing to do, it's pretty easy to implement. In the commandToProcess function, at libraries/process/System/Process/Internals.hs:455, the change is just

```
- return (cmd, translate cmd ++ "/c " ++ string)
+ return (cmd, translate cmd ++ "/c \"" ++ string ++ "\"")
```

(And in any case, the examples above should answer this amusing comment, immediately following those lines:

```
-- We don't want to put the cmd into a single
-- argument, because cmd.exe will not try to split it up. Instead,
-- we just tack the command on the end of the cmd.exe command line,
-- which partly works. There seem to be some quoting issues, but
-- I don't have the energy to find+fix them right now (ToDo). --SDM
-- (later) Now I don't know what the above comment means. sigh.
```

Later, Brandon comments: What that comment means is that how CMD.EXE handles spaces is Windows-version-dependent. On the other hand, I think it's mostly consistent between XP and Windows 7 --- and I feel sorry for anyone forced to use an older version.