simplify nscd lookup code for alt passwd/group backends

simplify nscd lookup code for alt passwd/group backends

previously, a sentinel value of (FILE *)-1 was used to inform the
caller of __nscd_query that nscd is not in use. aside from being an
ugly hack, this resulted in duplicate code paths for two logically
equivalent cases: no nscd, and "not found" result from nscd.

now, __nscd_query simply skips closing the socket and returns a valid
FILE pointer when nscd is not in use, and produces a fake "not found"
response header. the caller is then responsible for closing the socket
just like it would do if it had gotten a real "not found" response.