zc.buildout.download.Download may return temporary files
(only readable/writable by the current user) and by default
we'd like installed files to respect umask.

In the case of SlapOS, we usually want installed files to be readable
by members of 'slapsoft' group.

Not fully tested. With this commit, it is at least possible to
bootstrap SlapOS from Python 3.

- No more limit on the number of preseed parameters, by placing a preseed.cfg
  file inside the initrd, instead of passing them all via the command line.
  The kernel is usually limited to 32 parameters and it panics when there are
  too many.

- Dist-specific options.

- Recognize preseed aliases.

- late-command is run with '/bin/sh -e' and it must exit with EX_OK (0),
  otherwise the installer stops.

9p has no option to map uid/gid, which means that the user on the guest should
have the same ids than on the host to avoid issues when accessing mount points.

This is easier to create such user in 'vm.run' rather than in 'vm.install-*'.
In other words, this commit reverts to the previous behaviour that
'vm.install-*' does not create a normal user account by default.