Add a signal to a signal_set.

void add(
    int signal_number);
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void add(
    int signal_number,
    boost::system::error_code & ec);
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Add a signal to a signal_set with the specified flags.

void add(
    int signal_number,
    flags_t f);
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void add(
    int signal_number,
    flags_t f,
    boost::system::error_code & ec);
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