- Just like struct utmp, store strings inside struct utmpx itself. This
is needed to make things like pututxline() work.
- Add ut_id and ut_pid fields, even though they have little use in our
implementation.
- It turns out our "reboot" wtmp entries indicate a system boot, so
remove REBOOT_TIME
- Implement getutxline() and pututxline
- Add getutxuser() and setutxfile(), which allows us to crawl wtmp and
lastlog files as well.
- Add _ULOG_POSIX_NAMES, so we can already use the POSIX names if we
really want to.
