BUG/MEDIUM: httpclient: Rework CLI I/O handler to handle full buffer cases

BUG/MEDIUM: httpclient: Rework CLI I/O handler to handle full buffer cases

'httpclient' command does not properly handle full buffer cases. When the
response buffer is full, we exit to retry later. However, the context flags
are updated. It means when this happens, we may loose a part of the
response.

So now, flags are preserved when we fail to push data into the response
buffer. In addition, instead of dumping one part per call, we now try to
dump as much data as possible.

Finally, when there is no more data, because everything was dumped or
because we are waiting for more data from the HTTP client, the applet is
updated accordingly by calling applet_have_no_more_data(). Otherwise, when
some data are blocked, applet_putchk() already takes care to update the SE
flags. So, it is useless to call sc_need_room().

This patch should fix the issue #1723. It must be backported as far as
2.5. But a massive refactoring was performed in 2.6. So, for the 2.5 and
below, the patch will have to be adapted.