Add netconfig-commit.c whose main method, netconfig_commit actually sets
the configuration obtained by l_netconfig to the system netdev,
specifically it sets local addresses on the interface, adds routes to the
routing table, sets DNS related data and may add entries to the neighbor
cache. netconfig-commit.c uses a backend-ops type structure to allow
for switching backends. In this commit there's only a default backend
that uses l_netconfig_rtnl_apply() and a struct resolve object to write
the configuration.
netconfig_gateway_to_arp is moved from netconfig.c to netconfig-commit.c
(and renamed.) The struct netconfig definition is moved to netconfig.h
so that both files can access the settings stored in the struct.
