prefix_insert() and prefix_remove() emulate a tail queue by keeping the

prefix_insert() and prefix_remove() emulate a tail queue by keeping the
tail pointer (pointer to last element) around and depending on the state
of the list insert at head or insert after tailp. Now gcc has a hard time
to realize that the tail pointer is not used uninitalized. So rewrite the
code to be more explicit about tailp handling (also rename the pointer
to be more explicit). All in all this should be more readable and silences
the gcc warning as well.