Unify calling conventions for postgres/postmaster sub-main functions

There was a wild mix of calling conventions: Some were declared to
return void and didn't return, some returned an int exit code, some
claimed to return an exit code, which the callers checked, but
actually never returned, and so on.

Now all of these functions are declared to return void and decorated
with attribute noreturn and don't return.  That's easiest, and most
code already worked that way.

src/backend/postmaster/syslogger.c

@@ -139,6 +139,7 @@ static volatile sig_atomic_t rotation_requested = false;
 static pid_t syslogger_forkexec(void);
 static void syslogger_parseArgs(int argc, char *argv[]);
 #endif
+NON_EXEC_STATIC void SysLoggerMain(int argc, char *argv[]) __attribute__((noreturn));
 static void process_pipe_input(char *logbuffer, int *bytes_in_logbuffer);
 static void flush_pipe_input(char *logbuffer, int *bytes_in_logbuffer);
 static void open_csvlogfile(void);