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

boot.c

 /*-
  * Copyright (c) 2007 Yahoo!, Inc.
  * All rights reserved.
  * Written by: John Baldwin <jhb@FreeBSD.org>
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions
  * are met:
  * 1. Redistributions of source code must retain the above copyright
  *    notice, this list of conditions and the following disclaimer.
@@ skipping 37 matching lines @@
 
 static unsigned int boot_entry = 0;
 static uuid_t boot_uuid = GPT_ENT_TYPE_FREEBSD_BOOT;
 static const char *pmbr_path = "/boot/pmbr";
 static const char *gptboot_path = "/boot/gptboot";
 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);
 }
 
 static int
 gpt_find(unsigned int boot_entry, uuid_t *type, map_t **mapp)
 {
         map_t *gpt, *tbl, *map;
         struct gpt_hdr *hdr;
         struct gpt_ent *ent;
@@ skipping 88 matching lines @@
                 return 1;
         }
 #endif
         mbr = pmbr->map_data;
         nbytes = read(bfd, mbr->mbr_code, sizeof(mbr->mbr_code));
         if (nbytes < 0) {
                 warn("unable to read PMBR boot loader");
                 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.
         }
         close(bfd);
 
 #ifdef LINUX
         /* We use the syslinux GPT new protocol. This requires the GUID
            of the boot partition to be at offset 424 in the mbr. See the
            doc in the syslinux source: src/doc/gpt.txt */
         if (gpt_find(boot_entry, &boot_uuid, &gptboot) != 0) {
@@ skipping 122 matching lines @@
                         return 1;
                 }
 
                 r = boot(fd);
 
                 gpt_close(fd);
         }
 
         return r;
 }