BUG/MINOR: startup: set HAPROXY_CFGFILES in read_cfg

BUG/MINOR: startup: set HAPROXY_CFGFILES in read_cfg

load_cfg() is called only once before the first reading of the configuration
(we parse here only the global section). Then, before reading the rest of the
sections (second call of read_cfg()), we call clean_env(). As
HAPROXY_CFGFILES is set in load_cfg(), which is called only once, clean_env()
erases it. Thus, it's not longer shown in "show env" output.

To fix this, let's set HAPROXY_CFGFILES in read_cfg(). Like this in
master-worker mode it is set for master and for worker processes, as it was
before the refactoring.

This fix doesn't need to be backported as related to the latest master-worker
architecture change.