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


git-svn-id: svn+ssh://svn.freebsd.org/doc/head@46067 c2e8774f-c49f-e111-b436-862b2bbc8956
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. :-)


git-svn-id: svn+ssh://svn.freebsd.org/doc/head@46068 c2e8774f-c49f-e111-b436-862b2bbc8956