add PERL_USE_3ARG_SIGHANDLER macro

add PERL_USE_3ARG_SIGHANDLER macro

There are a bunch of places in core that do

   #if defined(HAS_SIGACTION) && defined(SA_SIGINFO)

to decide whether the C signal handler function should be declared with,
and called with, 1 arg or 3 args.

This commit just adds

   #if defined(HAS_SIGACTION) && defined(SA_SIGINFO)
   #  define PERL_USE_3ARG_SIGHANDLER
   #endif

Then uses the new macro in all other places rather than checking
HAS_SIGACTION and SA_SIGINFO. Thus there is no functional change; it just
makes the code more readable.

However, it turns out that all is not well with core's use of 1-arg
versus 3-arg, and the few commits will fix this.