Detach shared memory in Postmaster child if not needed

Prior to PG16, postmaster children would manually detach from shared memory
if it was not needed. However, this behavior was removed in fork mode in
commit aafc05d.
Detaching shared memory when it is no longer needed is beneficial, as
postmaster children (like syslogger) don't wish to take any risk of
accidentally corrupting shared memory. Additionally, any panic in these
processes will not reset shared memory.
The attached patch addresses this issue by detaching shared memory after
fork_process().
Best regard,
Rui Zhao