util: Make out_of_memory() call abort() instead of exit(EXIT_FAILURE).

util: Make out_of_memory() call abort() instead of exit(EXIT_FAILURE).

exit(EXIT_FAILURE) will make a monitoring process (the one created by
--monitor) think that it should exit.  But the most likely reason for
out_of_memory() to be called is a bug: probably, the process is trying
to allocate more memory than there is available address space, e.g.
something like malloc(-1).  So it's better, in my opinion, to call abort()
instead, so that the monitor process restarts the daemon and we are more
likely to stay alive and, in addition, get a core dump and a useful bug
report.

I decided to implement a new general-purpose function for this purpose in
case we run into other similar situations in the future.

(I haven't actually run into this problem in practice.  This commit is
just speculation about what is better behavior.)