this introduces tar extraction inside a chroot. This is done spawning a
new process. Instead of creating a new executable (difficult to find,
package etc...) it adds the concept of a multicall process (like busybox).
Now a new multicall command "extracttar" is added.
tar.ExtractTar executes this new command passing a pipe to send the
filesMap and the tar contents to stdin.
The incomplete "insecure link" checks are removed from the the tar
functions. Additionally these functions are now not exported and the
extraction directory is always the root directory.
Simone Gotti