Added tool to perform NAT type detection/classification

[bennylp]

With current implementation, it is rather hard to troubleshoot ICE negotiation failure. Having information about NAT type will help debugging such failure.

[bennylp]

Done in r1495:

• Added nat_detect.[h|c] in PJNATH library.
• New PJNATH API: pj_stun_detect_nat_type()
• New PJSUA-LIB API: pjsua_detect_nat_type()
• pjsua will automatically perform NAT detection upon startup, when STUN is enabled.
• Same for the WinCE sample application.

[bennylp]

Fixed bug with CHANGE-REQUEST values in r1496

[bennylp]

Added comments etc. in r1497

[bennylp]

Optimize to complete faster, in r1498 and r1499

[bennylp]

In r1501:

• Added callback to report NAT detection result.
• pjsua will also put NAT type information in SDP by default. This can be disabled by setting pjsua_config.nat_type_in_sdp

[bennylp]

In r1523:

• fixed bug that triggers assertion in the timer callback. Thanks Scott M Ober for reporting the bug.

[bennylp]

In r1524:

• fixed bug: in certain situations on_request_complete() callback doesn't release the mutex forever, causing application to hang.

[bennylp]

In r1526:

• fixed bug: pj_sock_close() is called more than once.
• Also implemented NAT detection callback in Symbian UA sample.

[bennylp]

in r1533:

• added new pjsua API pjsua_call_get_rem_nat_type() to retrieve the NAT type of remote endpoint. See pjsua.h for more info.

[bennylp]

Fixed in r1553:

• The NAT type investigation in incoming INVITE caused assertion error when the INVITE comes without SDP