vserver: Give all vservers a shared /var/cache/apt/archives dir.

vserver: Give all vservers a shared /var/cache/apt/archives dir.

Each vserver has write access to this dir, so any packages downloaded
can be shared by all. This saves download bandwidth, but more
importantly it saves storage size (upgrading all vservers at the same
time easily eats up gigabytes of space).

Since each vservers still has its own Release and Contents files and
checks gnupg signatures and hashes before installing the .debs, there
should not be a security issue.