curl_easy_setopt.3: document CLOSESOCKET* options

Pass a pointer that will be untouched by libcurl and passed as the first

argument in the opensocket callback set with \fICURLOPT_OPENSOCKETFUNCTION\fP.

(Option added in 7.17.1.)

.IP CURLOPT_CLOSESOCKETFUNCTION

Function pointer that should match the \fIcurl_closesocket_callback\fP

prototype found in \fI<curl/curl.h>\fP. This function gets called by libcurl

instead of the \fIclose(2)\fP or \fIclosesocket(2)\fP call when sockets are

closed (not for any other file descriptors). This is pretty much the reverse

to the \fICURLOPT_OPENSOCKETFUNCTION\fP option. \fIReturn 0 to signal success

and 1 if there was an error.  (Option added in 7.21.7)

.IP CURLOPT_CLOSESOCKETDATA

Pass a pointer that will be untouched by libcurl and passed as the first

argument in the opensocket callback set with

\fICURLOPT_CLOSESOCKETFUNCTION\fP.  (Option added in 7.21.7)

.IP CURLOPT_PROGRESSFUNCTION

Function pointer that should match the \fIcurl_progress_callback\fP prototype

found in \fI<curl/curl.h>\fP. This function gets called by libcurl instead of