git-daemon: Simplify dead-children reaping logic

git-daemon: Simplify dead-children reaping logic

Move almost all code out of the child_handler() into check_dead_children().
The fact that systemcalls get interrupted by signals allows us to
make the SIGCHLD signal handler almost a no-op by simply running
check_dead_children() right before waiting on poll().

In case some systems do not interrupt systemcalls upon signal receipt,
all zombies will eventually be collected before the next poll() cycle.

Signed-off-by: Stephen R. van den Berg <srb@cuci.nl>
Signed-off-by: Junio C Hamano <gitster@pobox.com>