Merge pull request #198 from mtomaschewski/ifsysctl

initial ifsysctl utils
client: do not complain about missed /etc/HOSTNAME
types: added an bool like tristate integer type

The purpose is to allow a default/unset setting in cases
where the values are unknown yet or inherited from others.
xml: added a integer node element creation utility
dbus: added missed ni_dbus_dict_get_int16,32,64
schema: use tristate for ethtool offload flags
schema: use tristate for ipv4/ipv6 protocol flags