BUG/MEDIUM: remove debugging code from systemd-wrapper

Kristoffer Grönlund reported that after my recent update to the
systemd-wrapper, I accidentely left the debugging code which
consists in disabling the fork :-(

The fix needs to be backported to 1.5 as well since I pushed it
there as well.
(cherry picked from commit a55bbc64d8272e4066a67b6d190ffebaff2b300a)
BUG/MEDIUM: http: adjust close mode when switching to backend

Commit 179085c ("MEDIUM: http: move Connection header processing earlier")
introduced a regression : the backend's HTTP mode is not considered anymore
when setting the session's HTTP mode, because wait_for_request() is only
called once, when the frontend receives the request (or when the frontend
is in TCP mode, when the backend receives the request).

The net effect is that in some situations when the frontend and the backend
do not work in the same mode (eg: keep-alive vs close), the backend's mode
is ignored.

This patch moves all that processing to a dedicated function, which is
called from the original place, as well as from session_set_backend()
when switching from an HTTP frontend to an HTTP backend in different
modes.

This fix must be backported to 1.5.
(cherry picked from commit 4e21ff9244aefa56bcf0793a9e07edba2c3c1960)