nlist.h: Re-allow direct use of nlist.n_name

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, 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.
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