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.