Patch: Make dab exec return exit code of programm called

[jmartin]

It find it useful to evaluate the exit code of the programs I call using dab exec.

To do this, we need to append the following to sub ve_exec in /usr/share/perl5/PVE/DAB.pm:

    waitpid( $pid, 0 );
    my $rc=$? >> 8;
    if ($rc != 0) {
        die "@cmd exited with status $rc";
    }

Note: I'm not a perl expert, so this could probably be done better.