sometimes tunables may not be present or accessible, e.g. when
net.ipv6.conf.all.disable_ipv6 = 1
mark such tunables as optional so tuner will not fail to load if for example v6 tunables are not available. Also add test for this (neigh_table_v4only_test) which disables IPv6 while verifying the v4 aspects of the tuner still work.
sometimes tunables may not be present or accessible, e.g. when
net.ipv6.conf.all.disable_ipv6 = 1
mark such tunables as optional so tuner will not fail to load if for example v6 tunables are not available. Also add test for this (neigh_table_v4only_test) which disables IPv6 while verifying the v4 aspects of the tuner still work.