Here are the issues with current os::Bsd::set_signal_handler()

#1 According to https://pubs.opengroup.org/onlinepubs/009695399/functions/sigaction.html "The storage occupied by sa_handler and sa_sigaction may overlap, and a conforming application shall not use both simultaneously.", however, our code assumes they use different storages, which just happens to work on BSD/Linux right now

#2 The "bool set_installed" in the API is unused and can/should be removed

#3 We should be reusing existing methods, such as "set_our_sigflags()" instead of repeating the same code

#4 Comments could use cleanup and be expanded for those not familiar with details of signals

#5 Not directly related, but we could add assert to check whether we re-set the same signal