RFR: 8272674: Logging missing keytab file in Krb5LoginModule

[Weijun Wang]

The "Key for the principal foobar at acme.com not available in /home/foobar/foobar.keytab" debug output does not contain enough information. The keytab file might be missing, or not readable, or does not contain the required key(s).

Please note that this debug info is only visible when `-Dsun.security.krb5.debug=true` is added on the command line.

Fix is trivial. No new regression test needed.

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Commit messages:
 - 8272674: Logging missing keytab file in Krb5LoginModule

Changes: https://git.openjdk.java.net/jdk/pull/5176/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=5176&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8272674
  Stats: 6 lines in 1 file changed: 6 ins; 0 del; 0 mod
  Patch: https://git.openjdk.java.net/jdk/pull/5176.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/5176/head:pull/5176

PR: https://git.openjdk.java.net/jdk/pull/5176