Improve the login.defs unknown item error message

Improve the login.defs unknown item error message

Closes #746

Only print the 'unknown item' message to syslog if we are
actually parsing a login.defs.  Prefix it with "shadow:" to make
it clear in syslog where it came from.

Also add the source filename to the console message.  I'm not
quite clear on the econf API, so not sure whether in that path we
will end up actually having the path, or printing ''.

Signed-off-by: Serge Hallyn <serge@hallyn.com>