tutorial: signals aren't used for the threaded resolver

libcurl is completely thread safe, except for two issues: signals and SSL/TLS

handlers. Signals are used for timing out name resolves (during DNS lookup) -

when built without c-ares support and not on Windows.

when built without using either the c-ares or threaded resolver backends.

If you are accessing HTTPS or FTPS URLs in a multi-threaded manner, you are

then of course using the underlying SSL library multi-threaded and those libs