pgsql: Add backendType to PGPROC, replacing isRegularBackend

Add backendType to PGPROC, replacing isRegularBackend

We can immediately make use of it in pg_signal_backend(), which
previously fetched the process type from the backend status array with
pgstat_get_backend_type_by_proc_number(). That was correct but felt a
little questionable to me: backend status should be for observability
purposes only, not for permission checks.

Reviewed-by: Nathan Bossart <nathandbossart(at)gmail(dot)com>
Reviewed-by: Bertrand Drouvot <bertranddrouvot(dot)pg(at)gmail(dot)com>
Reviewed-by: Chao Li <li(dot)evan(dot)chao(at)gmail(dot)com>
Discussion: https://www.postgresql.org/message-id/b77e4962-a64a-43db-81a1-580444b3e8f5@iki.fi

Branch
------
master

Details
-------
https://git.postgresql.org/pg/commitdiff/084e42bc7109673e46527b0a0f284edf539c3285

Modified Files
--------------
src/backend/access/transam/twophase.c | 2 +-
src/backend/storage/ipc/procarray.c | 4 ++--
src/backend/storage/ipc/signalfuncs.c | 5 +----
src/backend/storage/lmgr/proc.c | 4 ++--
src/backend/utils/activity/backend_status.c | 25 -------------------------
src/include/storage/proc.h | 5 +++--
src/include/utils/backend_status.h | 1 -
7 files changed, 9 insertions(+), 37 deletions(-)