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.
* Adds several links to documentation of library functions which were
missing.
* Marks documentation of deprecated library functions "(deprecated)".
* Removes spurious .html suffixes.
and receive data over a connection previously setup with curl_easy_perform()
and its CURLOPT_CONNECT_ONLY option. The sendrecv.c example was added to
show how they can be used.
