pgsql: Introduce a registry of built-in shmem subsystems

Introduce a registry of built-in shmem subsystems

To add a new built-in subsystem, add it to subsystemslist.h. That
hooks up its shmem callbacks so that they get called at the right
times during postmaster startup. For now this is unused, but will
replace the current SubsystemShmemSize() and SubsystemShmemInit()
calls in the next commits.

Reviewed-by: Ashutosh Bapat <ashutosh.bapat.oss@gmail.com>
Reviewed-by: Matthias van de Meent <boekewurm+postgres@gmail.com>
Reviewed-by: Daniel Gustafsson <daniel@yesql.se>
Discussion: https://www.postgresql.org/message-id/CAExHW5vM1bneLYfg0wGeAa=52UiJ3z4vKd3AJ72X8Fw6k3KKrg@mail.gmail.com

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/1fc2e9fbc0a3d17aa484dbfab11af58eb2cb20ad

Modified Files
--------------
src/backend/bootstrap/bootstrap.c       |  2 ++
src/backend/postmaster/launch_backend.c |  2 ++
src/backend/postmaster/postmaster.c     |  5 +++++
src/backend/storage/ipc/ipci.c          | 21 +++++++++++++++++++++
src/backend/storage/ipc/shmem.c         |  6 ++++--
src/backend/tcop/postgres.c             |  3 +++
src/include/storage/ipc.h               |  1 +
src/include/storage/subsystemlist.h     | 23 +++++++++++++++++++++++
src/include/storage/subsystems.h        | 30 ++++++++++++++++++++++++++++++
src/tools/pginclude/headerscheck        |  1 +
10 files changed, 92 insertions(+), 2 deletions(-)