Wipe out memory used for storing SSL keys before released

[nanang]

Zeroing our buffers should be sufficient as we cannot really manage the OpenSSL internal buffers. Moreover, it seems that OpenSSL already does wipe out its internal buffers, i.e: a lot of OpenSSL_cleanse() calls in OpenSSL source code, the function will fill a buffer with garbage or zero. But unfortunately cannot really find official docs about it.

Additionally, SSL socket pool content will be zeroed before released, it is done using a new API pj_pool_secure_release().

Thanks Peter Koletzki for the feedback.

[nanang]

In 5990:

Close #2179: Wipe out memory used for storing SSL keys before released.

[nanang]

In 5994:

Re #2179: Wipe out memory used for storing SSL keys in PJSIP TLS transport and pjsua app. Thanks Peter Koletzki for the feedback.

[nanang]

In 5999:

Re #2179: Fixed linking error when PJSIP_HAS_TLS_TRANSPORT is not defined.