core: Run startup/shutdown functions only once

Even if defined for multiple hosts. Startup or shutdown callbacks registered by any directive (startup, shutdown, markdown, git, log, etc.) will only run as many times as it appears in the Caddyfile, not repeated for each host that shares that server block. Fixing this involved refactoring three packages (yeesh) and we need to restore some tests that are no longer valid (that used to verify splitting a multiServerBlock into multiple serverBlocks).

server/server.go

@@ -83,18 +83,23 @@ func (s *Server) Serve() error {
 
 		// Execute shutdown commands on exit
 		if len(vh.config.Shutdown) > 0 {
-			go func() {
+			go func(vh virtualHost) {
+				// Wait for signal
 				interrupt := make(chan os.Signal, 1)
-				signal.Notify(interrupt, os.Interrupt, os.Kill) // TODO: syscall.SIGQUIT? (Ctrl+\, Unix-only)
+				signal.Notify(interrupt, os.Interrupt, os.Kill)
 				<-interrupt
+
+				// Run callbacks
+				exitCode := 0
 				for _, shutdownFunc := range vh.config.Shutdown {
 					err := shutdownFunc()
 					if err != nil {
-						log.Fatal(err)
+						exitCode = 1
+						log.Println(err)
 					}
 				}
-				os.Exit(0)
-			}()
+				os.Exit(exitCode) // BUG: Other shutdown goroutines might be running; use sync.WaitGroup
+			}(vh)
 		}
 	}