runtime: revert CL 18835; don't install new signal stack unconditionally on dragonfly

This change reverts CL 18835 which is a workaroud for older DragonFly
BSD kernels, and fixes #14051, #14052 and #14067 in a more general way
the same as other platforms except NetBSD.

This change also bumps the minimum required version of DragonFly BSD
kernel to 4.4.4.

Fixes #16329.

Change-Id: I0b44b6afa675f5ed9523914226bd9ec4809ba5ae
Reviewed-on: https://go-review.googlesource.com/29491Reviewed-by: Ian Lance Taylor <iant@golang.org>

src/runtime/os_dragonfly.go

@@ -201,16 +201,21 @@ func minit() {
 	_g_.m.procid = uint64(*(*int32)(unsafe.Pointer(&_g_.m.procid)))
 
 	// Initialize signal handling.
-
-	// On DragonFly a thread created by pthread_create inherits
-	// the signal stack of the creating thread. We always create
-	// a new signal stack here, to avoid having two Go threads
-	// using the same signal stack. This breaks the case of a
-	// thread created in C that calls sigaltstack and then calls a
-	// Go function, because we will lose track of the C code's
-	// sigaltstack, but it's the best we can do.
+	var st sigaltstackt
+	sigaltstack(nil, &st)
+	if st.ss_flags&_SS_DISABLE != 0 {
 		signalstack(&_g_.m.gsignal.stack)
 		_g_.m.newSigstack = true
+	} else {
+		// Use existing signal stack.
+		stsp := uintptr(unsafe.Pointer(st.ss_sp))
+		_g_.m.gsignal.stack.lo = stsp
+		_g_.m.gsignal.stack.hi = stsp + st.ss_size
+		_g_.m.gsignal.stackguard0 = stsp + _StackGuard
+		_g_.m.gsignal.stackguard1 = stsp + _StackGuard
+		_g_.m.gsignal.stackAlloc = st.ss_size
+		_g_.m.newSigstack = false
+	}
 
 	// restore signal mask from m.sigmask and unblock essential signals
 	nmask := _g_.m.sigmask