[Toybox] [CLEANUP] last part 1

[Rob Landley]

I keep writing up little notes to self and then forgetting to post 'em.
This was from my first pass over "last". (Not a huge amount of cleanup,
most of it was research.)

Currently, toys.h includes utmpx.h but not utmp.h. The reason is that
utmpx.h is in posix-2008, and utmp.h isn't:

  http://pubs.opengroup.org/onlinepubs/9699919799/basedefs/utmpx.h.html

It turns out, utmp.h is deprecated, replaced by utmpx.h:


http://lists.freebsd.org/pipermail/freebsd-current/2010-January/014893.html

And there's example code on how to use the new posix interface in the
programming book written by Michael Kerrisk (the man page maintainer), ala:

  http://man7.org/tlpi/code/online/dist/loginacct/dump_utmpx.c.html

It _also_ turns out that in Linux, struct utmp and utmpx are the same
thing. So the accessor functions changed, but the actual file format
didn't. Odd.

Let's gloss over that for the moment, and focus on cleaning up the code
that's there. First problem, my netbook doesn't list any logged in users:

  $ last
  wtmp begins Thu May  1 04:52:20 2014

Hmmm... how about if I do a fresh login:
  $ last
  root     tty4                Fri May  9 05:32   still logged in
  root     tty4                Fri May  9 05:32 - 05:32  (00:00)

  wtmp begins Fri May  9 05:32:02 2014

Ok, I can work with that.

Going through main():
  GLOBALS start zeroed so we don't need to assign NULL to TT.list.
  &tm[1] is just tm+1.

  Special-casing an empty file seems unnecessary. Also, instead of
  having an lseek before the loop and another at the end of the loop,
  let's have just one at the start of the loop. Between the two of
  those, the need for stat goes away.

  Well, the other thing the stat was doing was giving a timestamp
  for an empty log. Let's just use current time in that case.

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