From sgsn_vty.c, copy the cfg_grx_ggsn_cmd to add an ares server to the static
sgsn_instance.
This is sort of preliminary. As described in comments, the sgsn_ares functions
should actually be separated from the static sgsn structure. gtphub keeps such
an sgsn structure just for the sgsn_ares functions.
Sponsored-by: On-Waves ehi
gtphub_ext.c's initial purpose was to wrap a specific function. The file
then turned into everything related to DNS, which fits pretty well. Rename
to gtphub_ares.c.
Tweak the header comment to reflect the new file name.
Sponsored-by: On-Waves ehi