conf: convert network_conf.c to use g_auto* pointers

conf: convert network_conf.c to use g_auto* pointers

This was mostly boilerplate conversion, but in one case I needed to
define several differently named char* to take the place of a single
char *tmp that was re-used multiple times, and in another place there
was a single char* that was used at the toplevel of the function, and
then later used repeatedly inside a for loop, so I defined a new
separate char* inside the loop.

Signed-off-by: Laine Stump <laine@redhat.com>
Reviewed-by: Erik Skultety <eskultet@redhat.com>