[2.3][Process] Add possibility to use array prefixes

| Q             | A
| ------------- | ---
| Bug fix?      | no
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | no
| License       | MIT
| Doc PR        | no

This can be useful to use `'/opt/custom-php/bin/php' 'composer.phar'` as a prefix, it is not possible with the current implementation.

I submit the patch on 2.3 as `ProcessBuilder::setPrefix` method has been added for release 2.3. If it's too late for such modification, let me know, I'll open the PR against master.