5562: Add support for "Expect: 100-Continue" flow for PUT.

5562: Add support for "Expect: 100-Continue" flow for PUT.

This tells cURL to wait for a "go ahead" from the Keep server (in the form of a
HTTP/1.1 "100 Continue" response) instead of sending the request body
immediately.  This allows the server to reject the request if the request is
invalid or the server is read-only, without waiting for the client to send the
entire block.

Uses pycurl.UPLOAD to enable the 'PUT' and 'Expect: 100-continue" behavior.
Use INFILESIZE to sent Content-Length and use READFUNCTION to actually spool
the data.