fcm make: combine SHELL and OUT events

[matthewrmshin]

When a build task triggers a shell command, the shell utility generates a SHELL event when the command returns, and the task generates a separate OUT event to print the standard output and error. In a multi-process environment, these event may not be handled back-to-back and may get intermingled with other events. It would be desirable if the 2 events can be optionally combined.