This function extracts file descriptor information from a given multi_handle.
libcurl returns its fd_set sets. The application can use these to select() on,
but be sure to FD_ZERO them before calling this function as
curl_multi_fdset(3) only adds its own descriptors it doesn't zero or
otherwise remove any other. The curl_multi_perform(3) function should be
called as soon as one of them are ready to be read from or written to.
If no file descriptors are set by libcurl, max_fd will contain -1 when
this function returns. Otherwise it will contain the higher descriptor number
libcurl set.
You should also be aware that when doing select(), you should consider using a
rather small (single-digit number of seconds) timeout and call
curl_multi_perform regularly - even if no activity has been seen on the
fd_sets - as otherwise libcurl-internal retries and timeouts may not work as
you'd think and want.
Starting with libcurl 7.16.0, you should use curl_multi_timeout to
figure out how long to wait for action.