Find unreachable paragraphs and sections

Find paragraphs and sections that no path from the program's entry point reaches, by building a control flow graph over the procedure division and walking it. The graph follows fall-through between adjacent paragraphs, GO TO including DEPENDING ON, PERFORM including the full THRU range, and the branch targets registered by EXEC CICS HANDLE; it treats XCTL, RETURN and ABEND as ending the program. Programs containing ALTER are skipped, because ALTER retargets a GO TO at run time and leaves the program with no static control flow.