import-mbox.py should not start more threads than there are files
import-mbox.py should not start more threads than there are files

This fixes #209
jimjag/ponymail,Humbedooh/ponymail,jimjag/ponymail,Humbedooh/ponymail,Humbedooh/ponymail,quenda/ponymail,jimjag/ponymail,jimjag/ponymail,quenda/ponymail,quenda/ponymail