[Process] add a method to the ProcessBuilder to automatically prepend the executed script with "exec"

| Q             | A
| ------------- | ---
| Bug fix?      | no
| New feature?  | yes
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | 
| License       | MIT
| Doc PR        | symfony/symfony-docs#3999

This pull requests addresses the issue raised by @romainneutron in #5759. It eases the process of creating a ``Process`` whose script should be prepended by `exec`.
ProcessBuilder cannot create chained commands (commands separated by `;` or `&&`), it only allows single commands.

I don't know any equivalent on windows, but I don't think it's relevant on this platform: We would use `exec` to solve the issue mentioned in #5759 (**The sh wrapper** paragraph)

I think we could allow preprending on linux system that are not in sigchild environment. I've to admit this is a hacky way to solve this PHP issue, however it works very well and would allow using the ProcessBuilder when the use of `exec` is needed: There is no way to use this functionality at the moment: as the ProcessBuilder escapes all arguments and as `exec` is not a command (it must not be escaped), the only way is to build the command by hand, which is not something I would recommend.

I'd be in favor of such addition, with all the care and love suggested by @stof (windows and sigchild environments detection to avoid errors)