BUG/MINOR: mworker: fix typo &= instead of & in proc list serialization

BUG/MINOR: mworker: fix typo &= instead of & in proc list serialization

In mworker_proc_list_to_env(), a typo used '&=' instead of '&' when
checking PROC_O_TYPE_WORKER in child->options. This would corrupt the
options field by clearing all bits except PROC_O_TYPE_WORKER, but since
the function is called right before the master re-execs itself during a
reload, the corruption has no actual effect: the in-memory proc_list is
discarded by the exec, and the options field is not serialized to the
environment anyway.

This should be backported to all maintained versions.