Simplify and unify handler vectors in Posix signal code

In signal handling code, we have code sections which save signal handler state into vectors of sigaction structures or vectors of integers. All these code sections can be unified, disentangled and the using code simplified.

There are three places where we do this:

1) When installing hotspot signal handlers, should we find a handler in place and signal chaining is enabled, we save the original handler inside a sigaction array and a corresponding sigset.

2) if diagnostics are enabled with -Xcheck:jni, we periodically check if our hotspot signal handlers had been replaced (`static void check_signal_handler(int sig)`).

3) There is a complicated dance between VMError and the posix signal handler code: If a fatal error happens, we enter error reporting and install the secondary handler (`VMError::install_secondary_signal_handler()`). Before doing that, we store the handler we replace in yet another array, in this case one array for the handler address, one for the flag. I believe the purpose of this is to - when printing signal handlers as part of error reporting - print the original signal handler instead of the secondary crash handler (see `PosixSignals::print_signal_handler()`) and not to trip the error output in this function.