Use non-cryptographically-secure PRNG to generate a nonce for cookies.

Rationale: the nonce here is only used to make sure there is a low
probability of duplication, according to section B.2 of RFC7873.
It is only 32-bit, and even if an attacker knows the algorithm used
to generate nonces it won't, in any way, give him any platform to
attack the server as long as server secret used to sign the
(nonce, time) pair with HMAC-SHA1 is secure.

On the other hand, currently, each packet sent requires (unnecessarily)
a CS pseudo-random number which is ineffective.
27 jobs for wpk-non-crypto-secure-cookies in 8 minutes and 25 seconds (queued for 5 seconds)
Status Job ID Name Coverage
  Precheck
passed #77223
amd64 docker linux
precheck:debian:sid:amd64

00:00:20

 
  Build
passed #77224
amd64 docker linux
build:centos:centos6:amd64

00:02:55

passed #77225
amd64 docker linux
build:centos:centos7:amd64

00:03:04

passed #77226
amd64 docker linux
build:clang:debian:stretch:amd64

00:02:20

passed #77230
i386 docker linux
build:clang:debian:stretch:i386

00:02:23

passed #77227
amd64 docker linux
build:debian:jessie:amd64

00:02:12

passed #77229
amd64 docker linux
build:debian:sid:amd64

00:02:17

passed #77231
i386 docker linux
build:debian:sid:i386

00:01:26

passed #77228
amd64 docker linux
build:debian:stretch:amd64

00:02:17

passed #77232
amd64 docker linux
pkcs11:build:debian:sid:amd64

00:02:21

 
  Test
passed #77233
amd64 docker linux
docs:debian:sid:amd64

00:00:48

passed #77247
amd64 docker linux
install:debian:sid:amd64

00:01:46

passed #77249
amd64 docker linux
pkcs11:systemtest:debian:sid:amd64

00:04:41

passed #77248
amd64 docker linux
pkcs11:unittest:debian:sid:amd64

00:00:49

passed #77241
amd64 docker linux
systemtest:centos:centos6:amd64

00:04:51

passed #77242
amd64 docker linux
systemtest:centos:centos7:amd64

00:04:55

passed #77243
amd64 docker linux
systemtest:debian:jessie:amd64

00:04:57

passed #77245
amd64 docker linux
systemtest:debian:sid:amd64

00:04:44

passed #77246
i386 docker linux
systemtest:debian:sid:i386

00:04:47

passed #77244
amd64 docker linux
systemtest:debian:stretch:amd64

00:04:57

passed #77234
amd64 docker linux
unittest:centos:centos6:amd64

00:00:50

passed #77235
amd64 docker linux
unittest:centos:centos7:amd64

00:01:00

passed #77239
amd64 docker linux
unittest:clang:debian:stretch:amd64

00:00:43

passed #77236
amd64 docker linux
unittest:debian:jessie:amd64

00:00:49

passed #77238
amd64 docker linux
unittest:debian:sid:amd64

00:00:51

passed #77240
i386 docker linux
unittest:debian:sid:i386

00:00:53

passed #77237
amd64 docker linux
unittest:debian:stretch:amd64

00:00:53