Log error on failed underscores(), moving it to shared/

Log error on failed underscores(), moving it to shared/

Move underscores() to shared/. It's the same as alias_normalize(), but
it rather operates in place, with the same string being passed.

The difference now that it's in shared/ is that it's a non-logging
function.

This makes us a little bit more verbose: we don't accept partially
correct module and aliases names in kcmdline and in configuration files.
We log an error instead.