MFC r331949, r332437, r332438

r331949:
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.

r332427:
Check the return value of fseek.

r332438:
Remove a debugging printf that crept in.

Sponsored by: iXsystems, Inc.