Re-allow direct use of nlist.n_name in <nlist.h>

Re-allow direct use of nlist.n_name in <nlist.h>

Commit e8d340de ("Remove a.out support from nlist()") introduced a copy
of the definition of nlist from a.out.h. However, as well as having
n_name inside n_un, on the various BSDs n_name could also be accessed
as a direct member of nlist, and this is made use of by FreeBSD's
usr.bin/netstat/main.c. Thus we should also add the same enclosing
anonymous union.

[guillem@hadrons.org:
 - Add a minimal unit test. ]

Closes: !4
Signed-off-by: Guillem Jover <guillem@hadrons.org>