Change Emacs 'sort' API to use three-way comparison

[Paul Eggert <eggert@cs.ucla.edu>]

One infelicity I noticed in the recent change to 'sort' in the trunk is 
that the new implementation calls its predicate twice for each 
comparison.  This is because the Lisp API says the comparison function 
returns a boolean (nil or non-nil), whereas qsort_r wants the comparison 
function to return a ternary value (-1, 0, or 1). If the predicate is 
expensive, the new Fsort can be twice as slow as the old.  We could tune 
it but I don't see how to get it any faster than 1.5x slower than 
before, assuming random input and an expensive comparison function.

To fix this I propose changing the API for 'sort' so that its function 
argument is no longer a predicate, but instead returns a negative 
integer, 0, or a positive integer.  For compatibility with old code, it 
would treat nil as if it were nonpositive (thus requiring a reverse 
comparison) and noninteger nonnil values as if they were positive.  This 
wouldn't be 100% upward-compatible, because if an existing predicate 
returns a nonpositive integer to stand for 'true' there will be a silent 
change to behavior, but I expect such usage is so rare that we don't 
need to worry about it.