uloop: make SIGCHLD signal handling optional

Some programs want to manage their own child life cycle without using
SIGCHLD signal handler. In these cases, uloop is reaping children for
them because they don't have SIGCHLD handler set. This patch makes it
possible to disable reaping children through 'uloop_handle_sigchld'
variable.

Signed-off-by: Petar Paradzik <petar.paradzik@sartura.hr>