libudev-util: change util_replace_whitespace to return number of chars in dest

libudev-util: change util_replace_whitespace to return number of chars in dest

Instead of returning 0, which is unhelpful, return the number of chars
copied into the dest string.  This allows callers that care about that
to easily use it, instead of having to calculate the strlen.

No current users of the function check the return value, so this does not
break any existing code; it is used in the following patch.