process: fix interpreter support

[fischerling]

Previously, the argv array was not altered. However, this does not work when the script was executed using execp.
When executing an executable from the PATH the interpreter has no way to determine what file it should load.

Now we do the correct thing and add the path to the executed file as second argument shifting everything else to the right.

TODO: Pass the path to the interpreter as first argument.

Example

Imagine the script print_foo:

#!/bin/shell
echo foo

Previously it was only executable in the containing directory, or using an absolute path.

Now since the absolute path to the file is passed to the interpreter as second argument the shell can always use the absolute path to the script.