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
added dopen()/buflen() and optional options
emulating knetfile APIs
forgot to alias kurl_t to knetFile
forgot ";"
wrong dopen() interface
need to grow buffer

One multi_perform() frequently invokes multiple write_cb(). We have to use a
dynamic buffer. Hmm.. I really do not like the curl APIs, although I understand
why they are designed this way.