MINOR: mworker: restore initial env before wait mode

MINOR: mworker: restore initial env before wait mode

This patch is the follow-up of 1811d2a6ba (MINOR: tools: add helpers to
backup/clean/restore env).

In order to avoid unexpected behaviour in master-worker mode during the process
reload with a new configuration, when the old one has contained '*env' keywords,
let's backup its initial environment before calling parse_cfg() and let's clean
and restore it in the context of master process, just before it enters in a wait
polling loop.

This will garantee that new workers will have a new updated environment and not
the previous one inherited from the master, which does not read the configuration,
when it's in a wait-mode.