resume command generation every atom for hook use

[creidiki]

resume commands should be generated at the very end of each package's build (right before it moves onto the next package), so that it can be exported to hooks, with the idea of writing it to a file (to be overwritten every build) for resume functionality in case of power failure or hard lock.

for example, writing to ~/.paludis-resume after each build & cleanup etc is done, and deleting it when the whole command exits successfully.

[ciaranm]

r2733, thanks