daemon: use an argv_array to exec children

daemon: use an argv_array to exec children

Our struct child_process already has its own argv_array.
Let's use that to avoid having to format options into
separate buffers.

Note that we'll need to declare the child process outside of
the run_service_command() helper to do this. But that opens
up a further simplification, which is that the helper can
append to our argument list, saving each caller from
specifying "." manually.

Signed-off-by: Jeff King <redacted>