devmatch: do nothing if booted in safe mode

Safe mode may be enabled because of some problem discovered with a kmod,
thus wanting to avoid devmatch. Honor it and let the user deal with
the fallout of not autoloading modules. We've at least made it to
userland and the user demonstrably has a local console as they selected
safe mode in the first place, so this shouldn't be fatal if we don't,
e.g., load a necessary NIC driver.