Make dump's behaviour more sensible when the output file is a fifo.

Normally trewind() performs a close-open-close cycle to rewind the
tape when closing the device, but this is not ideal for fifos. We
now skip the final open-close if the output descriptor is a fifo.

PR:		bin/25474
Submitted by:	Alex Bakhtin <bakhtin@amt.ru>
MFC after:	1 week

sbin/dump/tape.c

@@ -43,6 +43,7 @@ static const char rcsid[] =
 #include <sys/socket.h>
 #include <sys/time.h>
 #include <sys/wait.h>
+#include <sys/stat.h>
 
 #include <ufs/ufs/dinode.h>
 #include <ufs/ffs/fs.h>
@@ -314,6 +315,7 @@ flushtape()
 void
 trewind()
 {
+	struct stat sb;
 	int f;
 	int got;
 
@@ -358,6 +360,10 @@ trewind()
 		return;
 	}
 #endif
+	if (fstat(tapefd, &sb) == 0 && S_ISFIFO(sb.st_mode)) {
+		(void)close(tapefd);
+		return;
+	}
 	(void) close(tapefd);
 	while ((f = open(tape, 0)) < 0)
 		sleep (10);