Add media transport preferences (STUN, ICE settings) to account setting (thanks Régis Montoya for the suggestion)

[bennylp]

With #539 it is now possible to configure media transport settings on per account basis. This is needed for example when application is supporting multiple accounts where one is connecting through Internet and the other is through VPN. The specification is as follows.

Configurations

STUN, ICE, and TURN configurations will be added to pjsua_acc_config with the default values values taken from the pjsua_config and pjsua_media_config. The settings in the pjsua_acc_config are the ones that will actually be used.

Contact header generation

The generation rule of the Contact header value doesn't change, apart from the fact that STUN setting is now account specific.

Via sent-by generation

Initial Via sent-by value now will be affected by STUN setting. Subsequent update of the value doesn't change; it follows the rules in #1537.

Media transports

STUN, ICE, and TURN setup of the media transports now will follow the account config settings instead.