When entering the chroot, cd into the port's source directory.
Fail if given SOURCE_DIR is not contained in archive.

* Instead of ignoring the problem and then failing in the next step, complain
  about the missing source dir. This additionally fixes the problem that
  retrying with a fixed SOURCE_DIR would require manual removal of the flag.unpack
  marker file.
Improve user experience in chroot shell.

* Activate system inputrc in chroot for improved navigational features.
* Create basic profile in order to show the current dir in the prompt.
Ask user what to do if unmounting failed repeatedly.

* Instead of failing, give the user the option to force-unmount
  the volume.
Minor improvements of output, suggested by Diver.

* Show full recipe path in output header for a specific port.
* Show full path of resulting packages in output footer.
Build the source package at a later stage.

* The source package is now built only after a successful build - this should reduce
  waiting times considerably when working on ports with large source trees.
Really stop the build after an interrupt.

* Before, only the child process was stopped, the parent did continue, which
  would now cause it to create the source package even after an interrupted
  build.