since time64 switchover has changed the size and layout of the struct
anyway, take the opportunity to fix it up so that it can be shared
between 32- and 64-bit ABIs on the same system as long as byte order
matches.

the ut_type member is explicitly padded to make up for m68k having
only 2-byte alignment; explicit padding has no effect on other archs.

ut_session is changed from long to int, with endian-matched padding.
this affects 64-bit archs as well, but brings the type into alignment
with glibc's x86_64 struct, so it should not break software, and does
not break on-disk format. the semantic type is int (pid-like) anyway.
the padding produces correct alignment for the ut_tv member on 32-bit
archs that don't naturally align it, so that ABI matches 64-bit.

this type is presently not used anywhere in the ABI between libc and
libc consumers; it's only used between pairs of consumers if a
third-party utmp library using the system utmpx.h is in use.