networkd: do not complain about IFF_RUNNING

Otherwise:
eth0: unknown link flags gained: 0x00040 (ignoring)

[tomegun: hiding these messages is ok, as IFF_RUNNING is redundant
          and can be deduced from operstate and IFF_LOWER_UP]
libnetworkd: add link local tests

- Also only allow positive ifindex on both dhcp and ipv4ll

[tomegun: the kernel always sets a positive ifindex, but some APIs accept
          ifindex=0 with various meanings, so we should protect against
          accidentally passing ifindex=0 along.]
sd-dhcp-client: log positive error number

Log error no for such client_stop(client, DHCP_EVENT_STOP)