sysprep: Don't remove /var/cache/apt/archives/partial directory.

Removing this directory breaks Ubuntu guests.

This change adds a utility function which removes only files from a
directory.  This is a safer way to clean cache directories etc.
sysprep: Fix documentation for --mount-options.
builder: Add Ubuntu 10.04LTS, 12.04LTS and 13.10 templates.
sysprep: ubuntu: Use secure SHA-512 crypt back to Ubuntu 10.04.
daemon: 'exists' API should not follow broken symlinks.

Using guestfs_exists on a symlink which existed but pointed to a
non-existent file was returning false.  However exists obviously
should not be following the symlink in the first place.
daemon: Refactor bind-mount code.

This is just code motion.
daemon: command: Copy appliance /etc/resolv.conf in before running commands.

When you try to run commands for an Ubuntu guest, they fail because in
Ubuntu /etc/resolv.conf is a symlink to /run/...  and this turns out
to be a dangling symlink when the Ubuntu guest is mounted up under the
appliance.

Therefore even if the network is enabled, any command which tries to
do name resolution will fail.

Ideally we would like to bind-mount the appliance /etc/resolv.conf
into the sysroot.  However this is not possible because mount is buggy
(see comment).  So instead we use a complex hack to achieve the same
ends.

Note this is only done if the network is enabled and if /etc in the
guest actually exists.  The original /etc/resolv.conf is restored
as soon as the command has run.
Version 1.23.28.