pam: don't leak TACACS+ secret to journald

If journald for syslog is used, the journal is going to store everything,
even DEBUG loglevel messages. Pre-journald logging of system-wide DEBUG
loglevel could be avoided and is not affected in all cases.

With journald presence it's probably safe to no longer log sensitive
details at DEBUG level.

Fix #149

Signed-off-by: Daniel Gollub <dgollub@att.com>