Teach DSM registry to retry entry initialization if needed.

Teach DSM registry to retry entry initialization if needed.

If DSM registry entry initialization fails, backends could try to
use an uninitialized DSM segment, DSA, or dshash table (since the
entry is still added to the registry).  To fix, restructure the
code so that the registry retries initialization as needed.  This
commit also modifies pg_get_dsm_registry_allocations() to leave out
partially-initialized entries, as they shouldn't have any allocated
memory.

DSM registry entry initialization shouldn't fail often in practice,
but retrying was deemed better than leaving entries in a
permanently failed state (as was done by commit 1165a933aa, which
has since been reverted).

Suggested-by: Robert Haas <[email protected]>
Reviewed-by: Robert Haas <[email protected]>
Discussion: https://postgr.es/m/E1vJHUk-006I7r-37%40gemulon.postgresql.org
Backpatch-through: 17