fixup! fixup! download: clean-up, fix, optimization

Commit c3903c7e was error-prone
because (lambda f1, f2: False) may be wrong in other cases than where
it's currently used.
16 jobs for master in 0 seconds
Status Job ID Name Coverage
  External
passed SlapOS.Buildout.UnitTest-Master.Python2

00:12:44

passed SlapOS.Buildout.UnitTest-Master.Python3

00:20:22

passed SlapOS.Buildout.UnitTest-Master.Python2

00:06:52

passed SlapOS.Buildout.UnitTest-Master.Python2

00:58:42

passed SlapOS.Buildout.UnitTest-Master.Python2

00:09:30

passed SlapOS.Buildout.UnitTest-Master.Python2

00:11:33

passed SlapOS.Buildout.UnitTest-Master.Python2

00:37:29

passed SlapOS.Buildout.UnitTest-Master.Python2

00:12:46

passed SlapOS.Buildout.UnitTest-Master.Python2

00:09:31

passed SlapOS.Buildout.UnitTest-Master.Python3

00:16:35

passed SlapOS.Buildout.UnitTest-Master.Python3

00:11:56

passed SlapOS.Buildout.UnitTest-Master.Python3

00:23:43

passed SlapOS.Buildout.UnitTest-Master.Python3

01:06:20

passed SlapOS.Buildout.UnitTest-Master.Python3

01:13:03

passed SlapOS.Buildout.UnitTest-Master.Python3

00:14:13

passed SlapOS.Buildout.UnitTest-Master.Python3

00:16:29