The libc will now use the file given by the 'nameserver_file' attribute
to get the DNS nameserver address instead of reading '/etc/resolv.conf'.
It defaults to '/socket/nameserver' which is the common location when
using the lxip VFS plugin. As a constraint the libc will read the first
line and expects the nameserver address without any keywords in front of
it.
Fixes#2861.
