IfAPI: Add some more accessors

Summary:

• if_setreassignfn for wireguard.
• if_getinputfn() and if_getstartfn() for various drivers. Use the function descriptor typedefs for these and the setters.
• vlantrunk accessor. This is used by VLAN_CAPABILITIES() used by several drivers, as well as directly by mxge(4).
• if_pcp member accessor, used by cxgbe.
• accessors for netmap adapter.

Sponsored by: Juniper Networks, Inc.
Reviewed By: glebius
Differential Revision: https://reviews.freebsd.org/D38202