Allow application to specify more than one STUN servers for more robustness, and continue application startup if STUN resolution fails

[bennylp]

Currently if STUN server is down, PJSUA-LIB initialization will fail. Having more than one STUN servers will increase the robustness of the application in this case.

Also it may be desirable to allow application startup to continue when STUN servers fail to be contacted. Some firewall with SIP ALG (notably Microsoft Forefront Threat Management Gateway) may decidedly block STUN traffic, since it can manipulate private address in SIP packet into public address.

[bennylp]

Done in r2864:

PJSUA-LIB:

• New fields in pjsua_config to specify more than one STUN servers (the stun_srv_cnt and stun_srv array)
• The existing stun_host and stun_domain fields are deprecated, but backward compatibility is maintained. If stun_srv_cnt is zero, the library will import the entries from stun_host and stun_domain
• The library will now resolve the STUN server entries one by one and test it before using it
• By default, PJSUA-LIB startup will continue even when STUN fails to be contacted. This behavior is configurable via the new pjsua_config.stun_ignore_failure setting (which by default is set to TRUE)
• New auxiliary API pjsua_resolve_stun_servers() to perform resolution and test against array of STUN servers

pjsua application:

• The --stun-srv command line options can now be specified more than once

[bennylp]

Some test data for future reference:

• --stun-srv stun.pjsip.org
• --stun-srv stun.pjsip.org:3478
• --stun-srv pjsip.org --nameserver 192.168.0.1
• --stun-srv pjsip.org:3478 --nameserver 192.168.0.1
• --stun-srv sip.pjsip.org --nameserver 192.168.0.1
• --stun-srv sip.pjsip.org:3478 --nameserver 192.168.0.1
• --stun-srv helloworld --stun-srv stun.pjsip.org
• --stun-srv 192.168.0.2 --stun-srv stun.pjsip.org
• --stun-srv 192.168.0.2 --stun-srv 192.168.0.3 --stun-srv stun.pjsip.org
• --stun-srv helloworld --stun-srv stun.pjsip.org --nameserver 192.168.0.1
• --stun-srv 208.109.222.137 --nameserver 192.168.0.1
• --stun-srv 208.109.222.137:3478 --nameserver 192.168.0.1
• --stun-srv stun.pjsip.org:12345 --stun-srv stun.pjsip.org:3478
• --stun-srv helloworld --stun-srv 192.168.0.2

[nanang]

In r2895:

• Fixed compile error caused by implicit conversion from void* in test_stun_on_status().