Automatically add '--' on Einhorn.upgrade_commandline
Put the reexec_commandline on State

This was a misunderstanding of mine: Einhorn doesn't pass the entire
command line through to its children; we have to keep it on State,
otherwise it gets lost on the second reload.
Place dropped env variables on Einhorn::State

...instead of deleting them from the startup environment. I
misunderstood how the re-execing works, and thought the re-exec'd
master process would get all the original command line arguments. Not
so: You have to place all the things that you want to survive a reload
onto State.

This means that --drop-env-var now survives the second restart,
too. Yay!