mainloop: capture output of short-lived init procs

mainloop: capture output of short-lived init procs

The handler for the signal fd will detect when the init process of a container
has exited and cause the mainloop to close. However, this can happen before the
console handlers - or any other events for that matter - are handled. So in the
case of init exiting we still need to allow for all buffered input to the
console to be handled before exiting. This allows us to capture output from
short-lived init processes.

This is conceptually equivalent to my implementation of ExecReaderToChannel()
https://github.com/lxc/lxd/blob/master/shared/util_linux.go#L527

Closes #1694.

Signed-off-by: Christian Brauner <christian.brauner@ubuntu.com>