BUG/MINOR: mworker: do not export HAPROXY_MWORKER_REEXEC across programs

BUG/MINOR: mworker: do not export HAPROXY_MWORKER_REEXEC across programs

This undocumented variable is only for internal use, and its sole
presence affects the process' behavior, as shown in bug #1324. It must
not be exported to workers, external checks, nor programs. Let's unset
it before forking programs and workers.

This should be backported as far as 1.8. The worker code might differ
a bit before 2.5 due to the recent removal of multi-process support.

(cherry picked from commit 3c032f2d4d3ba508d508ae70603ccac6d5cf9d4a)
Signed-off-by: Christopher Faulet <cfaulet@haproxy.com>
(cherry picked from commit a9274a17f6466b82359a5b6954bb3a35bce814f3)
Signed-off-by: Christopher Faulet <cfaulet@haproxy.com>