getdef: avoid spurious error messages about unknown configuration options

getdef: avoid spurious error messages about unknown configuration options

def_find can return NULL for unset, not just unknown, config options.  So
move the decision of whether to log an error message about an unknown config
option back into def_find, which knows the difference.  Only putdef_str()
will pass a char* srcfile to def_find, so only calls from putdef_str will
cause the message, which was the original intent of fa68441bc4be8.

closes #967

fixes: fa68441bc4be8 ("Improve the login.defs unknown item error message")
Signed-off-by: Serge Hallyn <serge@hallyn.com>