Fix elog(FATAL) before PostmasterMain() or just after fork().

Since commit 97550c0711972a9856b5db751539bbaf2f88884c, these failed with
"PANIC:  proc_exit() called in child process" due to uninitialized or
stale MyProcPid.  That was reachable if close() failed in
ClosePostmasterPorts() or setlocale(category, "C") failed, both
unlikely.  Back-patch to v13 (all supported versions).

Discussion: https://postgr.es/m/20241208034614.45.nmisch@google.com

src/backend/main/main.c

@@ -33,6 +33,7 @@
 
 #include "bootstrap/bootstrap.h"
 #include "common/username.h"
+#include "miscadmin.h"
 #include "port/atomics.h"
 #include "postmaster/postmaster.h"
 #include "storage/s_lock.h"
@@ -96,6 +97,7 @@ main(int argc, char *argv[])
 	 * localization of messages may not work right away, and messages won't go
 	 * anywhere but stderr until GUC settings get loaded.
 	 */
+	MyProcPid = getpid();
 	MemoryContextInit();
 
 	/*

src/backend/postmaster/fork_process.c

@@ -20,6 +20,7 @@
 #include <openssl/rand.h>
 #endif
 
+#include "miscadmin.h"
 #include "postmaster/fork_process.h"
 
 #ifndef WIN32
@@ -63,6 +64,7 @@ fork_process(void)
 	if (result == 0)
 	{
 		/* fork succeeded, in child */
+		MyProcPid = getpid();
 #ifdef LINUX_PROFILE
 		setitimer(ITIMER_PROF, &prof_itimer, NULL);
 #endif

src/backend/postmaster/postmaster.c

@@ -2649,7 +2649,7 @@ ClosePostmasterPorts(bool am_syslogger)
 
 
 /*
- * InitProcessGlobals -- set MyProcPid, MyStartTime[stamp], random seeds
+ * InitProcessGlobals -- set MyStartTime[stamp], random seeds
  *
  * Called early in the postmaster and every backend.
  */
@@ -2658,7 +2658,6 @@ InitProcessGlobals(void)
 {
 	unsigned int rseed;
 
-	MyProcPid = getpid();
 	MyStartTimestamp = GetCurrentTimestamp();
 	MyStartTime = timestamptz_to_time_t(MyStartTimestamp);