BUG/MINOR: mworker: mworker_reexec: unset MODE_STARTING before free startup logs...

BUG/MINOR: mworker: mworker_reexec: unset MODE_STARTING before free startup logs ring

Flag MODE_STARTING should be unset for master just before freeing the startup
logs ring, as it triggers the copy of process logs to this ring, see the code
of print_message().

Moreover with this flag set, if startup logs ring pointer is NULL, any
print_message() triggered just before the execvp in mworker_reexec() will call
startup_logs_init(). So ring will be allocated again "discretely" and after
execvp we will lost its address, as in step_init_1() we will call again
startup_logs_init().

No need to backport this fix as it's related to the latest master-worker
refactoring.