Unify several ways to tracking backend type

Add a new global variable MyBackendType that uses the same BackendType
enum that was previously only used by the stats collector.  That way
several duplicate ways of checking what type a particular process is
can be simplified.  Since it's no longer just for stats, move to
miscinit.c and rename existing functions to match the expanded
purpose.

Reviewed-by: Julien Rouhaud <rjuju123@gmail.com>
Reviewed-by: Kuntal Ghosh <kuntalghosh.2007@gmail.com>
Reviewed-by: Alvaro Herrera <alvherre@2ndquadrant.com>
Discussion: https://www.postgresql.org/message-id/flat/c65e5196-4f04-4ead-9353-6088c19615a3@2ndquadrant.com

src/include/pgstat.h

@@ -13,6 +13,7 @@
 
 #include "datatype/timestamp.h"
 #include "libpq/pqcomm.h"
+#include "miscadmin.h"
 #include "port/atomics.h"
 #include "portability/instr_time.h"
 #include "postmaster/pgarch.h"
@@ -712,25 +713,6 @@ typedef struct PgStat_GlobalStats
 } PgStat_GlobalStats;
 
 
-/* ----------
- * Backend types
- * ----------
- */
-typedef enum BackendType
-{
-	B_AUTOVAC_LAUNCHER,
-	B_AUTOVAC_WORKER,
-	B_BACKEND,
-	B_BG_WORKER,
-	B_BG_WRITER,
-	B_CHECKPOINTER,
-	B_STARTUP,
-	B_WAL_RECEIVER,
-	B_WAL_SENDER,
-	B_WAL_WRITER
-} BackendType;
-
-
 /* ----------
  * Backend states
  * ----------
@@ -1286,7 +1268,6 @@ extern const char *pgstat_get_wait_event_type(uint32 wait_event_info);
 extern const char *pgstat_get_backend_current_activity(int pid, bool checkUser);
 extern const char *pgstat_get_crashed_backend_activity(int pid, char *buffer,
 													   int buflen);
-extern const char *pgstat_get_backend_desc(BackendType backendType);
 
 extern void pgstat_progress_start_command(ProgressCommandType cmdtype,
 										  Oid relid);