Graceful handling on transport disconnection or registration failure

[bennylp]

Issues:

• PJSUA-LIB should re-register automatically on transport disconnection or registration failure (temporal failure).
• Calls disconnection after a re-registration attempt fails.

Thanks Klaus Darillion and Fabio Pietrosanti for the request.

[nanang]

In r3128:

• Added initial version of automatic re-registration after registration failure and automatic call disconnection after re-registration attempt fails.
• Published auto re-registration setting to pjsua app.
• Updated pjsip_regc_send() to retrieve the transport earlier (was only in tsx_callback()).
• Fixed TCP and TLS transport to prevent transport deletion in transport disconnection callback.
• Fixed wrong keep-alive settings used by TLS transport (was using TCP keep-alive settings).

[nanang]

(In [3139]) Minor fix #1044:

• Moved the code of disconnecting calls (after first re-reg attempt failure) to schedule_reregistration(), so it will be executed earlier (right after the failure of re-reg attempt).