Remove special handling of SIGCHLD

Use sigaction/3,4 for setting the SIGCHLD handler. The default action is
to ignore SIGCHLD, i.e., not notify the caller of SIGCHLD events.
Notifcation can be enabled by setting the handler to 'trap':

    SIGCHLD = alcove:define(Drv, 'SIGCHLD'),
    alcove:sigaction(Drv, SIGCHLD, trap).