Main is now HscMain: this will transform eventually into the
compiler's entry point.

CmdLineOpts now separates flags into static flags and dynamic flags;
dynamic flags will be passed around explicitly and can therefore
change from compilation to compilation.  All the dump flags and a few
language flags (like -fglasgow-exts) are currently DynFlags.

NOTE: nothing will now compile, for a long while until we get all this
connected up.  This commit is so that we start pulling in the same
direction.