Restore the API of the kf_sa_local and kf_sa_peer members.

In 11.x and earlier these were accessible as direct members of 'struct
kinfo_file'. Existing code already knows about the new location of
these members as well, so wrapper macros did not work for these
fields. Instead, define an anonymous struct containing the fields
from 'struct kinfo_file' in FreeBSD 11 that were not part of the
'kf_un' union. This anonymous struct is then placed in an anonymous
union along with the new 'kf_un' union. This preserves the API of
both structure layouts without requiring any wrapper macros.

PR: 231525
Reviewed by: kib
Approved by: re (gjb)
Differential Revision: https://reviews.freebsd.org/D17262