Create argv before fork

[straight-shoota]

Reduces code executing between fork and exec. Most importantly, removes the unsafe allocation from command_args.map.

[ysbaddaden]

Now I'm confused with command and prepared_args 😵

[straight-shoota]

prepared_args are the exact arguments to execvp, i.e. already transformed to C types.
command is usually the same value as prepared_args[1][0], but as a String instead of a C string (UInt8*).

Actually, it should probably better be LibC::Char*.