DrvAPI: Extend driver KPI with more accessors

Summary:
Add the following accessors to hide some more netstack details:

• if_get/setcapabilities2 and *bits analogue
• if_setdname
• if_getxname
• if_transmit - wrapper for call to ifp->if_transmit()
• This required changing the existing if_transmit to

if_transmit_default, since that's its purpose.

• if_getalloctype
• if_getindex
• if_foreach_addr_type - Like if_foreach_lladdr() but for any address family type. Used by some drivers to iterate over all AF_INET addresses.
• if_init() - wrapper for ifp->if_init() call
• if_setinputfn
• if_setsndtagallocfn
• if_togglehwassist

Reviewers: transport, network, glebius, melifaro

Reviewed by: network, melifaro
Sponsored by: Juniper Networks, Inc.
Differential Revision: https://reviews.freebsd.org/D37664