Resolve hostname when trying to get the network interface

[riza]

Currently, pjsip will use default routing (from OS) when trying to get network interface.
In a multihomed environment, sometimes this is not the desired behavior.
This ticket will add an option to resolve hostname when trying to get the network interface to be put in the Via or Contact header.

To enable this option set resolve_hostname_to_get_interface from the pjsip_cfg_t::endpt to PJ_TRUE or
define PJSIP_RESOLVE_HOSTNAME_TO_GET_INTERFACE as PJ_TRUE on the config_site.h.

Thanks to Alexander Klyushin for the report.

[riza]

In 4802:

Re #1752: Add option to resolve hostname when trying to get network interface.