Multi-threaded applictions basically MUST set CURLOPT_NO_SIGNAL to 1L to
avoid the risk of getting a SIGPIPE.
Either way, a multi-threaded application that uses libcurl/openssl needs
to have a signhandler for or ignore SIGPIPE on its own.
Based on discussions in #2800Closes#2904
Because the old OpenSSL link now redirects to their master documentation
(currently 1.1.0), which does not document the required actions for
OpenSSL <= 1.0.2.
This is a new document to consolidate our thread safety information from
several documents (curl-www:features, libcurl.3, libcurl-tutorial.3).
Each document's section on multi-threading will now point to this one.
