[IOT-1313, IOT-1300] Improve random number generation

[IOT-1313, IOT-1300] Improve random number generation

Update ocrandom.c to use platform specific, secure random number
generators instead of rand(). Update the API in ocrandom.h, and make
corresponding updates to callers. Avoid using FP arithmetic in
OCGetRandomRange.

Change-Id: Ic0476dc69f1c649c4a07a46b99643cf7206f83af
Signed-off-by: Greg Zaverucha <gregz@microsoft.com>
Reviewed-on: https://gerrit.iotivity.org/gerrit/15243
Tested-by: jenkins-iotivity <jenkins-iotivity@opendaylight.org>
Reviewed-by: Kevin Kane <kkane@microsoft.com>
Reviewed-by: Dan Mihai <Daniel.Mihai@microsoft.com>