BUG/MEDIUM: applet: Properly handle receives of size 0

BUG/MEDIUM: applet: Properly handle receives of size 0

when appctx_rcv_buf() function was called to get data from the applet, but
to get zero bytes, nothing was performed and the function early
returned. However, we must at least take care to set SE_FL_WANT_ROOM if
necessary. Otherwise, if data are still blocked in the applet's output
buffer while the EOI/EOS are pending, the information can be reported to the
upper layer and remaining data can be lost.

Indeed, in such case, SE_FL_WANT_ROOM flag is here to specify the applet has
more data to deliver. Thanks to this flag, the stream will wait before
closing. But when appctx_rcv_buf() function is called, this flag is removed by
the stconn. It is the function responsibility to set it again when necessary.

This patch should fix second part of the issue #3366. It must be backported
to 3.0.