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

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 a regular file, now it is possible to do e.g.

$ kldxref -d /boot/kernel/ufs.ko

to see only ufs.ko metadata.

Reviewed by: emaste
Sponsored by: The FreeBSD Foundation
MFC after: 1 week
Differential revision: https://reviews.freebsd.org/D30368