sys: Remove ancient SCCS tags.

Remove ancient SCCS tags from the tree, automated scripting, with two
minor fixup to keep things compiling. All the common forms in the tree
were removed with a perl script.

Sponsored by: Netflix

IfAPI: Hide the macros that touch ifnet members

Nothing should be directly touching the ifnet members, which are hidden
in <net/if_private.h>, so hide them in the same header to avoid errors
from users.

Sponsored by: Juniper Networks, Inc.

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

ifnet/API: Move struct ifnet definition to a <net/if_private.h>

Hide the ifnet structure definition, no user serviceable parts inside,
it's a netstack implementation detail. Include it temporarily in
<net/if_var.h> until all drivers are updated to use the accessors
exclusively.

Reviewed by: glebius
Sponsored by: Juniper Networks, Inc.
Differential Revision: https://reviews.freebsd.org/D38046