sd-rtnl: link - allow setting the change mask
DEFINE_STRING_TABLE_LOOKUP: return _INVALID_* rather than assert on NULL string
sd-rtnl: add sd_rtnl_message_append_u16()
networkd: add basic bonding support

Refactor bridging support to be generic netdev support and extend it to
cover bonding as well.
sd-rtnl: simplify sd_rtnl_message_addr_new()

Split out into sd_rtnl_message_addr_set_{prefixlen,flags,scope}().