Fallback to real root

[jtsymon]

We use a fakechroot so we can write to /.
Files that we don't want to write to (such as apt-get library / config folders) can be linked in to the fakechroot.
However this requires manually doing so on a case-by-case basis whenever things break.
There are also problems with scripts which have hardcoded #!/usr/bin/interpreter, which does not exist in the fakechroot (but do exist at that location on the real root).

A possibly better solution would be as follows:

• When reading a file, use the file from the fakechroot if it exists, else fallback to the real root.
• When writing to a file, create the file on the fakechroot if it is not already on the fakechroot, then write to that file.

Problems:

• fakechroot does not appear to support this functionality (would need to find/write a tool that supports this).
• No way to "delete" a file that exists on the real root.

[jtsymon]

Fixed by 69b26a0