spawn: try to keep signals blocked in spawned child
While we only want to stop our daemons and gracefully destroy
subprocesses, it is common for 'Ctrl-C' from a terminal to kill
the entire pgroup.
Killing an entire pgroup nukes subprocesses like git-upload-pack
breaks graceful shutdown on long clones. Make a best effort to
ensure git-upload-pack processes are not broken when somebody
signals an entire process group.
Followup-to: commit 37bf2db81bbbe114d7fc5a00e30d3d5a6fa74de5
("doc: systemd examples should only kill one process")