[2.3][Process] Fix escaping on Windows

| Q             | A
| ------------- | ---
| Bug fix?      | yes
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| License       | MIT

Windows escaping is broken since the last merges.

After digging more on Windows escaping, I realised some things: 
 - We forbid environment variable expansion by escaping `%APPDATA%` to `^%"APPDATA"^%`
 - We explicitly ask for variable expansion at runtime (running the command line with the [`/V:ON`](https://github.com/symfony/symfony/blob/2.3/src/Symfony/Component/Process/Process.php#L235) flag). Running a command containing `!APPDATA!` will be escaped and expanded (our previous rule is easily overriden)
 - On platform that are not windows, we use strong escaping that prevents any variable expansion (`$PATH` will be escaped to `'$PATH'` that is not interpreted as the current PATH)

We have three possibilities:
 - Keep this behavior as this.
 - Prefer a consistent API and use a strong escaping strategy everywhere, but it would result in a BC break (see #8975).
 - Allow environment variable expansion and escape `%APPDATA%` to `"%APPDATA%"`

Any thoughts about this ?