Remove the mandatory "per-dust-output" fee rule.

Instead, enforce a rule similar to Bitcoin:  Completely disallow (no matter what fee) outputs less than 0.001 NMC.

This helps to harmonise fee rules between Namecoin and what is common in Bitcoin, which simplifies development of things like the Armory port.  (No special rules required for Namecoin.)  Dust spam as it appeared historically (and was prevented by the now-removed rule) should be avoided by the dust output rule just as well.