revert unrelated change that slipped into last commit

fix regression in tcsetattr on all mips archs

revert commit 8c316e9e49d37ad92c2e7493e16166a2afca419f. it was wrong
and does not match how the kernel API works.

fix spurious errors from pwd/grp functions when nscd backend is absent

for several pwd/grp functions, the only way the caller can distinguish
between a successful negative result ("no such user/group") and an
internal error is by clearing errno before the call and checking errno
afterwards. the nscd backend support code correctly simulated a
not-found response on systems where such a backend is not running, but
failed to restore errno.

this commit also fixed an outdated/incorrect comment.

avoid sending huge names as nscd passwd/group queries

overly long user/group names are potentially a DoS vector and source
of other problems like partial writes by sendmsg, and not useful.

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.

support alternate backends for the passwd and group dbs

when we fail to find the entry in the commonly accepted files, we
query a server over a Unix domain socket on /var/run/nscd/socket.
the protocol used here is compatible with glibc's nscd protocol on
most systems (all that use 32-bit numbers for all the protocol fields,
which appears to be everything but Alpha).