Stop using O_EXCL by default when opening the device for writing.

boot.c

Index: boot.c
diff --git a/boot.c b/boot.c
index 36b3560ddf1f23101fabc5c3b691030388d5fd6d..b1856db118f1c064a984ab75de24359f73928878 100644
--- a/boot.c
+++ b/boot.c
@@ -55,9 +55,8 @@ static u_long boot_size;
 static void
 usage_boot(void)
 {
-
 	fprintf(stderr,
-	    "usage: %s [-b pmbr] [-g gptboot] [-s count] device ...\n",
+	    "usage: %s [-b pmbr] [-g gptboot] [-i index ] [-s count] device ...\n",
 	    getprogname());
 	exit(1);
 }
@@ -166,7 +165,7 @@ boot(int fd)
 		return 1;
 	}
 	if (nbytes != sizeof(mbr->mbr_code)) {
-                warnx("short read of PMBR boot loader code (%ld/%ld bytes)",
+                warnx("short read of PMBR boot loader code (%zd/%zd bytes)",
                       nbytes, sizeof(mbr->mbr_code));
                 // not returning, this is usually okay.
 	}