Add the etdump utility for dumping El Torito boot catalog information.

This can be used to check existing images but will be used in the future to
find EFI ESP images placed in El Torito catalogs so they can be used for
hybrid boot purposes.

Reviewed by: imp (code), sbruno (man page), bcr (man page)
Sponsored by: iXsystems, Inc.
Differential Revision: https://reviews.freebsd.org/D14952