Move create_temp_file() out of #ifdef ENABLE_CRYPTO

Move create_temp_file() out of #ifdef ENABLE_CRYPTO

By using get_random() instead of prng_bytes(), we no longer have to place
create_temp_file() inside #ifdef ENABLE_CRYPTO.

The resulting filename now has 62 bits of entropy (2 * [0-INT_MAX])
instead of the previous 128 bits, but that should be plenty.  Assuming an
int is 32 bits, we would need about 2**31 (2147483648) files to have a
(roughly) 0.5 chance of failing in one of the 6 attempts we do.

(This is preparing to move the function out of misc.c, where I'd prefer to
not have to add a #include "crypto.h".)

Signed-off-by: Steffan Karger <steffan.karger@fox-it.com>
Acked-by: David Sommerseth <davids@openvpn.net>
Message-Id: <20170725210234.5673-1-steffan@karger.me>
URL: https://www.mail-archive.com/openvpn-devel@lists.sourceforge.net/msg15146.html
Signed-off-by: David Sommerseth <davids@openvpn.net>
(cherry picked from commit cd5a74d0d7c6347b31e261e98ca8984819e594df)