Allow on-detach callbacks for dynamic shared memory segments.

Just as backends must clean up their shared memory state (releasing
lwlocks, buffer pins, etc.) before exiting, they must also perform
any similar cleanups related to dynamic shared memory segments they
have mapped before unmapping those segments.  So add a mechanism to
ensure that.

Existing on_shmem_exit hooks include both "user level" cleanup such
as transaction abort and removal of leftover temporary relations and
also "low level" cleanup that forcibly released leftover shared
memory resources.  On-detach callbacks should run after the first
group but before the second group, so create a new before_shmem_exit
function for registering the early callbacks and keep on_shmem_exit
for the regular callbacks.  (An earlier draft of this patch added an
additional argument to on_shmem_exit, but that had a much larger
footprint and probably a substantially higher risk of breaking third
party code for no real gain.)

Patch by me, reviewed by KaiGai Kohei and Andres Freund.
Fix compiler warning.

get_user_name returns const char *, but we were assigning the result
to a char * variable.