Use the new shmem allocation functions in a few core subsystems

Use the new shmem allocation functions in a few core subsystems

These subsystems have some complicating properties, making them
slightly harder to convert than most:

- The initialization callbacks of some of these subsystems have
  dependencies, i.e. they need to be initialized in the right order.

- The ProcGlobal pointer still needs to be inherited by the
  BackendParameters mechanism on EXEC_BACKEND builds, because
  ProcGlobal is required by InitProcess() to get a PGPROC entry, and
  the PGPROC entry is required to use LWLocks, and usually attaching
  to shared memory areas requires the use of LWLocks.

- Similarly, ProcSignal pointer still needs to be handled by
  BackendParameters, because query cancellation connections access it
  without calling InitProcess

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