generators: define custom main func definer and use it where applicable

generators: define custom main func definer and use it where applicable

There should be no functional difference, except that the error message
is changd from "three or no arguments" to "zero or three arguments". Somehow
the inverted form always seemed strange.

umask() call is also dropped from run-generator. I think it wasn't dropped in
053254e3cb215df3b8c905bc39b920f8817e1c7d because the run generator was merged
around the same time.