Mark two signal flags as sig_atomic_t in pgbench and pg_test_fsync

Two booleans used for timeout tracking were used within some SIGALRM
signal handlers, but they were not declared as sig_atomic_t, so mark
them as such.  This has no consequence on WIN32 for both tools.

Author: Ranier Vilela
Reviewed-by: Michael Paquier, Masahiko Sawada
Discussion: https://postgr.es/m/CAEudQArCDQQiPiFR16=yu9k5s2tp4tgEe1U1ZbkW4ofx81AWWQ@mail.gmail.com

src/bin/pg_test_fsync/pg_test_fsync.c

@@ -68,7 +68,7 @@ static char full_buf[DEFAULT_XLOG_SEG_SIZE],
 		   *filename = FSYNC_FILENAME;
 static struct timeval start_t,
 			stop_t;
-static bool alarm_triggered = false;
+static sig_atomic_t alarm_triggered = false;
 
 
 static void handle_args(int argc, char *argv[]);

src/bin/pgbench/pgbench.c

@@ -310,7 +310,7 @@ const char *progname;
 
 #define WSEP '@'				/* weight separator */
 
-volatile bool timer_exceeded = false;	/* flag from signal handler */
+volatile sig_atomic_t timer_exceeded = false;	/* flag from signal handler */
 
 /*
  * We don't want to allocate variables one by one; for efficiency, add a