With this patch, the 'libc_lwip_nic_dhcp' plugin provides the DNS server
address acquired by lwIP via DHCP in the file '/etc/resolv.conf'.
This feature can be disabled from the config file:
<libc resolv="no" />
The static network interface configuration attributes are now also a part
of the '<libc>' config node:
<libc ip_addr="..." netmask="..." gateway="..." />
Fixes#731.
