Re-add domain/proxy code to fix linphone_account_creator_create_proxy_config

[danielfinke]

We are using linphone_account_creator_create_proxy_config as follows:

linphone_account_creator_set_password(account_creator, password);
linphone_account_creator_set_domain(account_creator, domain);
linphone_account_creator_set_display_name(account_creator, display_name);
linphone_account_creator_set_transport(account_creator, linphone_transport_parse(transport));

LinphoneProxyConfig* proxyCfg = linphone_account_creator_configure(account_creator);

However, since commit b41b920, linphone_proxy_config_check will fail because proxyCfg's reg_proxy parameter is NULL. Also _get_identity is unable to produce a valid identity.

I believe this also resolves the issue: BelledonneCommunications/linphone-iphone#247