[MEDIUM] signals: add support for registering functions and tasks

The two new functions below make it possible to register any number
of functions or tasks to a system signal. They will be called in the
registration order when the signal is received.

    struct sig_handler *signal_register_fct(int sig, void (*fct)(struct sig_handler *), int arg);
    struct sig_handler *signal_register_task(int sig, struct task *task, int reason);
6 files changed