declare perl core's sig handler as 1-arg

First some background:

UNIXy OSes support two types of signal handler function:

    Signal_t handler1(int sig);
    Signal_t handler3(int sig, siginfo_t *info, void *uap);

The original one-argument handler was set using the signal(2) system
call. The newer sigaction(2) system call allows either a 1-arg or
3-arg handler to be specified:

    act.sa_handler = handler1;
    sigaction(sig, act, NULL);

    act.sa_sigaction = handler3;
    act.sa_sa_flags |= SA_SIGINFO;
    sigaction(sig, act, NULL);

The current behaviour in perl core is that, in the presence of
HAS_SIGACTION and SA_SIGINFO, the signal handler type and function are
both declared as 3-arg, but perl still tells the kernel that the
supplied signal handler function takes one arg. This means that whenever
the kernel calls the handler, args 2 and 3 are whatever garbage the OS
and architecture cause them to happen to be.

Note that POSIX.xs *does* allow a 3-arg signal handler to be specified
by passing the SA_SIGINFO flag, and a couple of tests check for this.

Recently, gcc-8 has (quite reasonably) been warning that we're passing
around 3-arg function pointers where a 1-arg function pointer is
expected.

After the groundwork laid down by the previous commits in this branch,
this commit flips things over so that the perl core now declares its
handlers and handler type as being 1-arg, thus reflecting the reality
that the core has actually being using 1-arg handlers.

This makes a whole bunch of compiler noise like this go away:

    perl.h:2825:51: warning: cast between incompatible function types
    from ‘__sighandler_t’ {aka ‘void (*)(int)’} to ‘void (*)(int,
    siginfo_t *, void *)’ {aka ‘void (*)(int,  struct <anonymous> *,
    void *)’} [-Wcast-function-type]

In theory this should make no functional difference. It might cause
3rd-party XS modules to emit compiler warnings now if they are relying
on things like Sighandler_t to represent a 3-arg function - this commit
flips it to being 1-arg.

Author: iabyn

ext/POSIX/POSIX.xs

@@ -3096,7 +3096,6 @@ sigaction(sig, optaction, oldaction = 0)
 #endif
                           )
                         : (  oact.sa_handler   == PL_csighandler1p
-                          || oact.sa_handler   == PL_csighandlerp /* XXX tmp */
 #ifndef PERL_USE_3ARG_SIGHANDLER
                           || oact.sa_handler   == PL_csighandlerp
 #endif

perl.h

@@ -2795,7 +2795,13 @@ typedef struct padname PADNAME;
 
 
 #if defined(HAS_SIGACTION) && defined(SA_SIGINFO)
-#  define PERL_USE_3ARG_SIGHANDLER
+    /* having sigaction(2) means that the OS supports both 1-arg and 3-arg
+     * signal handlers. But the perl core itself only fully supports 1-arg
+     * handlers, so don't enable for now.
+     * NB: POSIX::sigaction() supports both.
+     *
+     * # define PERL_USE_3ARG_SIGHANDLER
+     */
 #endif
 
 /* Siginfo_t: