(closes #78)

This is a precaution to avoid saving sensitive data to disk, e.g. session keys.
We expect that corefiles are not world readable, but they're still sitting on
the physical disk and it's safer just to disable creating them.

GitHub issue #71 deals with a similar concern.

(per Anders Kaseorg)

In particular, srand( time( NULL ) ) is very predictable.

[keithw@mit.edu -- modified to use byte PRNG from /dev/urandom]

[Keegan: various small changes]

m4_ifnblank requires autoconf 2.64, and doesn’t seem necessary here.

Submitted upstream to autoconf-archive (no response yet):
https://savannah.gnu.org/patch/?7741



Signed-off-by: Anders Kaseorg <andersk@mit.edu>