Updated patches for mount, add umount

[Justus Winter]

Hi,

this is a patch series with all the non-trivial changes I posted this
week. This obsoletes:

1372686414-25363-1-git-send-email-4winter@informatik.uni-hamburg.de
1372930812-22762-1-git-send-email-4winter@informatik.uni-hamburg.de

This also adds a umount utility that should implement all of the
functionality used by Debians initscript package. Note that without a
mtab or /proc/mounts file the umount utility is not very useful, but I
used it successfully in combination with my mtab prototype to umount
nested tmpfs translators.

Cheers,
Justus