Teach growfs's dbg_open() that a filename of "-" for output means to

open "/dev/stdout".  This doesn't actually affect growfs, but does affect
ffsinfo, permitting ffsinfo to output to the shell's stdout rather than
requiring it be dumped to a file or explicitly pointed at a special
device.

Reviewed by:	peter

sbin/growfs/debug.c

@@ -71,7 +71,10 @@ void
 dbg_open(const char *fn)
 {
 
-	dbg_log=fopen(fn, "a");
+	if (strcmp(fn, "-") == 0)
+		dbg_log=fopen("/dev/stdout", "a");
+	else
+		dbg_log=fopen(fn, "a");
 
 	return;
 }