aq(4): expand and correct offloads, fix VLAN/multicast filtering

Advertise the offloads the hardware already performs, correct the TX
descriptor's L3 family selection, and correct the VLAN and multicast
receive-filter paths.

Offloads: advertise IFCAP_HWCSUM_IPV6 (adding CSUM_IP6_TCP/UDP/TSO to
isc_tx_csum_flags) and IFCAP_VLAN_HWTSO, and enable the RX outer
(S-VLAN) tag parse mode in aq_hw_offload_set().

TX descriptor L3 family: aq_setup_offloads() derived tx_desc_cmd_ipv4
from CSUM_IP|CSUM_TSO, but CSUM_TSO is (CSUM_IP_TSO|CSUM_IP6_TSO) and
tcp_output() sets both bits without regard to address family, so an
IPv6 TSO frame matched on CSUM_IP_TSO and went out with the IPv4
header-checksum command set on a frame that carries no IPv4 header.
The checksum flags cannot distinguish the family; key the bit off
IPI_TX_IPV4 instead, which iflib derives from the parsed ethertype,
as the IPI_TX_INTR test below it already does. Plain IPv6 checksum
offload was unaffected, as CSUM_IP6_TCP alone never matched the mask.

RX VLAN tag stripping: ring init hardwired hardware tag stripping off
while the RX path still set M_VLANTAG and the writeback tag for every
tagged frame, so a tagged frame arrived with the tag in line while the
mbuf claimed it stripped and ether_demux() parsed four bytes short of
the payload. Program per-ring stripping from IFCAP_VLAN_HWTAGGING and
set M_VLANTAG only under the same capability, so the two states stay
coherent.

VLAN filter and promiscuous edge cases: filter only when 1..16 VLANs are
registered -- with none (or more than the 16 the table holds) fall back
to VLAN-promiscuous and pass all tags, rather than dropping every tagged
frame against an empty filter table; and keep VLAN-promiscuous set
whenever the interface is IFF_PROMISC, so adding or removing a VLAN under
promisc does not clear it and start dropping tagged frames.

Multicast reconcile: ifdi_multi_set is declarative, but aq_if_multi_set()
only added -- shrinking the list left accept-all-multicast latched or
stale exact slots enabled, defeating hardware multicast filtering until
a reinit. Clear the exact slots before reprogramming the current list,
and always drive accept-all-multicast from the current state so a shrink
clears it.

Reviewed by: adrian
Differential Revision: https://reviews.freebsd.org/D58145