Re-register automatically on TCP/TLS transport disconnection

[bennylp]

PJSUA-LIB should re-register automatically once it detects that the underlying TCP/TLS transport connection has been disconnected.

Thanks Klaus Darillion and Fabio Pietrosanti for the request.

Specifications:

1. Network status will be monitored by application, so in the case of IP address changed, application should be better restarting pjsua.
2. Add new field in pjsua_acc_config, i.e: reg_retry_interval to configure the interval of auto re-register when registration failed of timeout or transport disconnection.
3. Add new field in pjsua_acc, i.e: route_down, a flag to specify whether the transport used for contacting the first route of the account is down.
   • If this flag is set, sending SIP messaging via PJSUA-LIB API (except the auto re-register) will be blocked (by returning an error code) immediately.
   • This flag will be set/unset automatically by PJSUA-LIB, i.e: set in the initialization or registration success, and unset in transport disconnected.
   • This flag is only usable when account has route-set (global proxy or account specific proxy is set).