MEDIUM: startup: split sending oldpids_sig logic for standalone and mworker modes

MEDIUM: startup: split sending oldpids_sig logic for standalone and mworker modes

Before refactoring the master-worker mode, in all runtime modes, when the new
process successfully parsed its configuration and bound to sockets, it sent
either SIGUSR1 or SIGTERM to the previous one in order to terminate it.

Let's keep this logic as is for the standalone mode. In addition, in standalone
mode we need to send the signal to old process before calling set_identity(),
because in set_identity() effective user or group may change. So, the order is
important here.

In case of master-worker mode after refactoring, master terminates the previous
worker by itself up to receiving "READY" status from the new one in
_send_status(). Master also sets at this moment HAPROXY_LOAD_SUCCESS env
variable and checks, if there are some other workers to terminate with
max_reloads exceeded.

So, now in master-worker mode we terminate old workers only, when the new one
has successfully done all initialization steps and has sent "READY" status to
master.