When NetBSD boots on mfs, all ports stop in single-user mode. Is there a
   particular reason for it ?


first off, s/mfs/md/.  mfs is a userland process that allocates some VM,
newfs's it, and mounts it as an ffs.  the in-kernel "memory disk" is a
different beast.


anyway, the issue here is that the these /sbin/init programs have been
compiled -DSMALL (see distrib/utils/x_init, i believe), and that enables
a very simple init that doesn't do multi user mode at all (i think).