This patch adds a flag -ddump-mod-cycles to the "ghc -M" dependency analyser.

The effect of
	ghc -M -ddump-mod-cycles
is to dump a list of cycles foud in the module graph.  The display is
trimmed so that only dependencies within the cycle are shown; and the
list of modules in a cycle is itself sorted into dependency order, so that
it is easy to track the chain of dependencies.

Open question: should the flag be "-ddump-mod-cycles" or "-optdep-dump-mod-cycles"?  For this reason I have not yet added to the documentation.