MEDIUM: startup: remove MODE_MWORKER_WAIT

MEDIUM: startup: remove MODE_MWORKER_WAIT

MODE_MWORKER_WAIT becames redundant with MODE_MWORKER, due to moving
master-worker fork in init(). This change allows master no longer perform
reexec just after forking in order to free additional memory.

As after the fork in the master process we set 'master' variable, we can
replace now MODE_MWORKER_WAIT in some 'if' statements by simple check of this
'master' variable.

Let's also continue to get rid of HAPROXY_MWORKER_WAIT_ONLY environment
variable, as it's no longer needed as well.

In cfg_program_postparser(), which is used to check if cmdline is defined to
launch a program, we completely remove the check of mode for now, because
the master process does not parse the configuration for the moment. 'program'
section parsing will be reintroduced in master later in the next commits.