tweak tail merging logic

tweak tail merging logic

 TAIL-ANNOTATE used MERGE-TAIL-CALLS policy (which is an integer) as a
 boolean, instead of comparing it to zero. Oops. This means that we always did
 TCO when possible.

 Since adding a debug-catch-tag effectively prevents TCO, and the policy
 controlling that looks just like what we would like to have in
 MERGE-TAIL-CALLS if TAIL-ANNOTATE were to use it correctly... just deprecate
 MERGE-TAIL-CALLS instead.