udev-rules: perform whitespace replacement for symlink subst values

udev-rules: perform whitespace replacement for symlink subst values

If the string_escape option is either unset or 'replace' (i.e. if it is
not 'none'), then enable whitespace replacement in SYMLINK variable
substitution values, as added in the last patch.

This will keep any whitespace that is directly contained in a SYMLINK
value, but will replace any whitespace that is added to the SYMLINK
value as a result of variable substitution (except $result/%c).

This fixes bug 4833.