knetfile-like interface to CURL
better interface and documentations
better CLI

sftp connection is really slow. Don't know why.
Fixed slow sftp/scp connection

In curl/docs/examples/fopen.c, the developer didn't check maxfd set by
curl_multi_fdset() and supposed that calling select(0,...) is effectively
equivalent to sleep. While the comment is correct, timeout estimated by
curl_multi_fdset() is frequently too large - in my case 10 seconds. We seldom
need to wait that long.

In curl_multi_fdset.3, the cURL developers recommended to wait at least 100ms
if maxfd is set to 1. This is what I am doing in the revised code. It does
little harm.
use larger buffer for remote files