Renamed lfs_entry_t -> lfs_mattr_t

Attributes are used to describe more than just entries, so calling these
list of attributes "entries" was inaccurate. However, the name
"attributes" would conflict with "user attributes", user-facing
attributes with a very similar purpose. "user attributes" must be kept
distinct due to differences in binary layout (internal attributes can
use a more compact tag+buffer representation, but expecting users to
jump through hoops to get their data to look like that isn't very
user-friendly).

Decided to go with "mattr" as shorthand for "meta-attributes", similar
to "metadata".