Remove programlistings that are inside other elements and replace them
based on the context they appear in with <screen><userinput> tags.
Also, remove three separate screen tags that belong together and
wrap them into one.  The output is not changed by these changes.

Obtained from:	    igor(1) output and rereading the FDP primer
Change the "hello world" examples so that they return 0.
I realize that this is not a C programming primer, but
we can at least try to be correct here. :-)