io/FileDescriptor: fold CreateSignalFD() into the SignalFD constructor

src/system/SignalFD.cxx

@@ -11,8 +11,15 @@
 void
 SignalFD::Create(const sigset_t &mask)
 {
-	if (!fd.CreateSignalFD(&mask))
+	int new_fd = ::signalfd(fd.Get(), &mask, SFD_NONBLOCK|SFD_CLOEXEC);
+	if (new_fd < 0)
 		throw MakeErrno("signalfd() failed");
+
+	if (!fd.IsDefined()) {
+		fd = UniqueFileDescriptor{new_fd};
+	}
+
+	assert(new_fd == fd.Get());
 }
 
 int