locale.c: Make locale collation predictions adaptive

locale.c: Make locale collation predictions adaptive

We try to avoid calling strxfrm() more than needed by predicting its
needed buffer size.  This generally works because the size of the
transformed string is roughly linear with the size of the input string.
But the key word here is "roughly".  This commit changes things, so that
when we guess low, we change the coefficients in the equation to guess
higher the next time.