BUG/MEDIUM: hlua: Fix getline() for TCP applets to work with applet's buffers

BUG/MEDIUM: hlua: Fix getline() for TCP applets to work with applet's buffers

The commit e5e36ce09 ("BUG/MEDIUM: hlua/cli: Fix lua CLI commands to work
with applet's buffers") fixed the TCP applets API to work with applets using
its own buffers. Howver the getline() function was not updated. It could be
an issue for anyone registering a CLI commands reading lines.

This patch should be backported as far as 3.0.

(cherry picked from commit c0ecef71d7ceaf404bc18241bdb878a4e72033b4)
Signed-off-by: Christopher Faulet <cfaulet@haproxy.com>