EXAMPLES: maintain haproxy 2.8 retrocompatibility for lua mailers script

EXAMPLES: maintain haproxy 2.8 retrocompatibility for lua mailers script

Because of b58bd97 ("MINOR: hlua_fcn/mailers: handle timeout mail from
mailers section"), latest examples/lua/mailers.lua file wouldn't work
if loaded with haproxy < 2.8.2 (which is not yet released), unless it
is manually recompiled from the latest code sources.

But this can be an issue given that direct URL may be referenced in
guides/tutorials as download link for the the script, independently
from the related haproxy source code revision.
ie: http://git.haproxy.org/?p=haproxy-2.8.git;a=blob_plain;f=examples/lua/mailers.lua;hb=HEAD

Thus, in order to make examples/lua/mailers.lua work with previous 2.8
versions, we ensure that the script stays compatible with pre-b58bd97
code by adding extra checks in the script.

This must be backported in 2.8 with b58bd97