[IOT-1519] Securely zero buffers containing secret data

[IOT-1519] Securely zero buffers containing secret data

Add an OICClearMemory helper function, and use it to securely
clear buffers that contain keys and other secret data that
shouldn't be left in the stack or on the heap.

Rename privateKey to g_privateKey in csr.c.

Fix a couple of leaked payloads on error return paths in
secureresourceprovider.c (which will also now zero their
contents).

Change-Id: If79c840ad758be2a7ca1bf7e6ccccb6dbdc39cf2
Signed-off-by: Kevin Kane <kkane@microsoft.com>
Reviewed-on: https://gerrit.iotivity.org/gerrit/14091
Tested-by: jenkins-iotivity <jenkins-iotivity@opendaylight.org>
Reviewed-by: Uze Choi <uzchoi@samsung.com>
Reviewed-by: Randeep Singh <randeep.s@samsung.com>