readutmp: fix non-portable UT_PID use

readutmp: fix non-portable UT_PID use

The module readutmp is broken for the standard use

  read_utmp(..., READ_UTMP_USER_PROCESS | READ_UTMP_CHECK_PIDS)

for all releases of FreeBSD until 8.3, and all OpenBSD ever released.

The reason is that those systems do not provide `utmp.ut_pid', thus
making the macro UT_PID(u) identical to naught, and then turning
the predicate

   UT_PID (u) <= 0

found in desirable_utmp_entry(), into a permanent true clause.
This makes desirable_utmp_entry() discard every legitimate
user's UTMP entry for said BSD releases. NetBSD and DragonflyBSD
are not touched by this malfunction.

Copyright-paperwork-exempt: yes

* lib/readutmp.c (desirable_utmp_entry) <READ_UTMP_CHECK_PIDS>:
  Use `UT_PID (u) > 0' as absolute condition.