Deparse system-BLOCK correctly

Deparse system-BLOCK correctly

Same with exec.  Don’t put ‘do’ before the block.

This was accomplished by running it through the same code path
(indirop) that print and sort already go through.

There was a slight discrepancy between listop and indirop in the way
they handled no arguments.  Before we would get:

    () = system;
    () = exec;
    () = sort();

Instead of making the first two get the redundant parentheses, I
changed indirop, so sort lost them.