booleans.
* libmisc/utmp.c: Add brackets and parenthesis.
* libmisc/utmp.c: Exit with the EXIT_FAILURE status code in case
of failure.
* libmisc/utmp.c: Avoid assignments in comparisons.
* lib/prototypes.h, libmisc/utmp.c: Change setutmp()'s prototype
to return an int because pututline() and pututxline() may fail.