klog: make krb5_524 non-fatal for native K5 tokens

The krb5_524_conv_principal() function should fail whenever the Kerberos
v5 principal cannot safely be mapped onto a Kerberos v4 principal, and
does fail on some Kerberos v5 principals used in real-world AFS
deployments.

Prior to this patchset a failure was treated as a fatal error that
in turn prevents an AFS token from being generated or set into the
cache manager.

Prior to b1f9b4cb5dd295162ae51704310e9d6058008f0a the
krb5_524_conv_principal() function wasn't used and a local client
mapping was created.  b1f9b4cb5dd295162ae51704310e9d6058008f0a
replaced the local mapping with the krb5 function because the local
mapping could be wrong and confusing.

The krb5_524_conv_principal() function as applied to AFS tokens is
just a local guess.  How the username in the token is interpreted by
the AFS server is up to the server.

krb5_524_conv_principal() is only used for Krb5 native tokens. For Krb4
tokens the krb5_524_convert_creds() function is used to obtain both the
Kerberos v4 ticket and the converted names from the KDC. Many
organizations used the krb524d service to perform name translation. When
the krb524d service is used, the name translation is performed by the KDC,
so there is no local call to krb5_524_conf_principal() which might fail.
As a result, disallowing the use of a native Krb5 token due to a failed
local name translation is a needless loss of functionality; the local name
translation is not an essential part of obtaining a token.

This patchset modifies the behavior such that krb5_524_conv_principal()
errors are non-fatal.

 1. If -noprdb is not specified the error message is generated
    and a NULL username is used.

 2. If the username is NULL the prdb lookup is disabled.

 3. If the username is NULL the informational messages do not
    include a username.

 4. If the username is NULL the username info provided to the
    cache manager in the token description is the nul string.

Credit to Ben Kaduk for assistance with the wording of this
commit message.

Change-Id: Ib07131fc0ff4bf5319815213198c3f0adac17b10
Reviewed-on: http://gerrit.openafs.org/11542
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Benjamin Kaduk <kaduk@mit.edu>
Reviewed-by: D Brashear <shadow@your-file-system.com>