daemon: use sigaction() to install child_handler()

daemon: use sigaction() to install child_handler()

Replace signal() with an equivalent invocation of sigaction(), but
make sure to NOT set SA_RESTART so the original code that expects
to be interrupted when children complete still works as designed.

This change has the added benefit of using BSD signal semantics reliably
and therefore not needing the rearming call in the signal handler.

Signed-off-by: Carlo Marcelo Arenas Belón <carenas@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>