BUG/MEDIUM: cache: Be sure to end the forwarding when XFER length is unknown

BUG/MEDIUM: cache: Be sure to end the forwarding when XFER length is unknown

This bug exists in the HTX code and in the legacy one. When the body length is
unknown, the applet hangs. For the legacy code, it hangs because the end of the
cached object is not correctly handled and the applet is never recalled. For the
HTX code, only the begining of the response (the 1st buffer) is sent then the
applet hangs. To work in HTX, The fast forwarding must be correctly handled.

This patch must be backported to 1.9.

[cf: the patch adding the function channel_add_input must be backported with
this one. It does not exist in 1.8 because only responses with a C-L are cached.]