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.