On UN*X, don't tell the client why authentication failed.

"no such user" tells the client that the user ID isn't valid and,
therefore, that it needn't bother trying to do password cracking for
that user ID; just saying that the authentication failed dosn't give
them that hint.

This resolves the third problem in Include Security issue F11: [libpcap]
Remote Packet Capture Daemon Multiple Authentication Improvements.

The Windows LogonUser() API returns ERROR_LOGON_FAILURE for both cases,
so the Windows code doesn't have this issue.  Just return the same
"Authentication failed" message on Windows to the user.

For various authentication failures *other* than "no such user" and
"password not valid", log a message, as there's a problem that may need
debugging.  We don't need to tell the end user what the problem is, as
they may not bother reporting it and, even if they do, they may not give
the full error message.