Removed isprefix() and replaced it with strncmp().

Removed isprefix() and replaced it with strncmp().
isprefix() had been ssequal(), which had not been what it advertised to be.
Now, all this is gone.
We could get problems if one of the arguments is NULL, as isprefix()
handled this situation, but strncmp() does not. It appears as if this case
does not show up in the code.