Part 2/2 of SF patch #416704: More robust freeze, by Toby Dickenson.

(With slight cosmetic improvements to shorten lines and a grammar fix
to a docstring.)

This addes -X and -E options to freeze.  From the docstring:

-X module     Like -x, except the module can never be imported by
              the frozen binary.

-E:           Freeze will fail if any modules can't be found (that
              were not excluded using -x or -X).