CURLOPT_NOSIGNAL(3) on macOS

The text b1668fc7a3e0, as macOS 10.12.5, macOS 10.12.4, macOS 10.12.3, macOS 10.12.2, macOS 10.12.1, macOS 10.12 shipped it.

skip all signal handling

CURLOPT_NOSIGNAL(3) on macOS now