kldxref: do not error out if specified path is not directory, for -d mode

[kib]

Summary

kldxref(8) is the only tool that can dump FreeBSD kernel module metadata, with the -d option. But the command line requirements for that are inconvenient, since parser requires that argv[1] is a directory containing whole set of modules to generate xref file.

For -d, allow argv[0] to be the file, now it is possible to do e.g.
$ kldxref -d /boot/kernel/ufs.ko
to see only ufs.ko metadata.

usr.sbin/kldxref/kldxref.c

main(int argc, char *argv[])
         }
         if (argc - optind < 1)
                 usage();
         argc -= optind;
         argv += optind;
 
         if (stat(argv[0], &sb) != 0)
                 err(1, "%s", argv[0]);
-        if ((sb.st_mode & S_IFDIR) == 0) {
+        if ((sb.st_mode & S_IFDIR) == 0 && !dflag) {
                 errno = ENOTDIR;
                 err(1, "%s", argv[0]);
         }
 
         ftsp = fts_open(argv, fts_options, compare);
         if (ftsp == NULL)
                 exit(1);