Replace graceful restart code for binary upgrades (SIGUSR2)

[mholt]

In #807 @tpng did the excellent thing of making all reloads, even graceful ones, happen within the same process. It deleted a lot of code and made Caddy much better.

However, now it is impossible, I think, to upgrade the Caddy binary gracefully. Where before a new caddy process would be executed, thus allowing the sysadmin to replace the old caddy binary with a new one, now that no longer happens. So you have to stop and restart the process to upgrade the server.

I propose we introduce the USR2 signal for graceful upgrades. This essentially restarts the process gracefully like we did before, except that we do not load a new Caddyfile; instead we pass in the current Caddyfile via gob.

Let's not begin work on this until I push my 0.9 progress.