Delay KeyboardInterrupt() handling when running a subprocess

Delay KeyboardInterrupt() handling when running a subprocess

Because we weren't giving subprocess time to gracefully exit, we'd
often get "Device or resource is busy" errors when CTRL+C'ing mkosi
as we we'd try to unmount directories while a subprocess was still
running in it. By delaying handling of SIGINT while a subprocess is
running, the subprocess can exit gracefully which prevents EBUSY errors
from occurring when running cleanup code. To allow interrupting hanging
subprocesses, we only delay the first SIGINT we receive. If a user
presses CTRL+C again, we exit immediately as before.