Detach shared memory in Postmaster child if not needed

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