Fix: Improve handling of soft signals on darwin

Fixes a bug on OSX where, if the debugged process spawned a child, when
that process received a SIGCHLD it would cause Delve to hang.

Fixes #197

_fixtures/sigchldprog.go

@@ -0,0 +1,25 @@
+package main
+
+import (
+	"bufio"
+	"fmt"
+	"log"
+	"os/exec"
+)
+
+func main() {
+	cmd := exec.Command("date")
+	reader, err := cmd.StdoutPipe()
+	if err != nil {
+		log.Fatalln(err)
+	}
+	scanner := bufio.NewScanner(reader)
+	go func() {
+		for scanner.Scan() {
+			fmt.Println(scanner.Text())
+		}
+		reader.Close()
+	}()
+	cmd.Start()
+	cmd.Wait()
+}