Make it easy to detach completely from shared memory.

The new function dsm_detach_all() can be used either by postmaster
children that don't wish to take any risk of accidentally corrupting
shared memory; or by forked children of regular backends with
the same need.  This patch also updates the postmaster children that
already do PGSharedMemoryDetach() to do dsm_detach_all() as well.

Per discussion with Tom Lane.