Use appropriate little-endian/big-endian types in struct definitions.

Use appropriate little-endian/big-endian types in struct definitions.

On-disk struct definitions used native types (u16/u32/u64/s16/s32/s64),
which doesn't say anything about the intended interpretation of the
data. The intention of having little-endian-specific types and
big-endian-specific types must have been to clarify interpretation of
data and intentions in the code. Therefore it seems reasonable to use
these types in struct definitions to clarify what data represention is
used to encode field data.

Because some struct members in layout.h are big-endian, this change also
means moving the duplicated definitions for big-endian byteswapping
macros and big-endian types found in acls.h and security.h to the
appropriate locations in endians.h and types.h respectively in order to
make them available for the struct definitions in layout.h.