caddy.go:569: could be simplified
sigtrap_posix.go:87: value of inst is never used
upgrade.go:151: should omit nil check; len() for nil slices is defined as zero
* SIGUSR2 triggers graceful binary upgrades (spawns new process)
* Move some functions around, hopefully fixing Windows build
* Clean up a couple file closes and add links to useful debugging thread
* Use two underscores in upgrade env var
To help ensure uniqueness / avoid possible collisions
