analyze: don't claim "@known" was an unlisted syscall
    
    It's a sycall group of our own definition, and the output is erroneous
    to claim otherwise. Let's hide it.
    
    This adds syscall_set_add() which is nicely symmetric to the existing
    syscall_set_remove().
    
    Follow-up for: 6d6a08547c03f96dc798cda1ef4a8d3013d292d5