Allow "copy-to-repl" from *sldb* buffers from other threads.

The previous change didn't deal with the edge case where the *sldb*
buffer is associated with a thread different from the REPL's. In these
cases the *, / and + variables weren't being set in the REPL where
they're useful. This change fixes that by forcing the reset to
correct values in the REPL thread.

* contrib/slime-repl.el (slime-repl-eval-string): Call
SWANK:SET-AUTO-REPL-VARS after acknowledgement from
SWANK:LISTENER-EVAL.

* contrib/swank-repl.lisp (repl-eval): Extract setting * et al to
SWANK:SET-AUTO-REPL-VARS.
(set-auto-repl-vars): New function. Also sets *LAST-REPL-VALUES*
and *LAST-REPL-FORM*.
(*last-repl-values*, *last-repl-form*): new variables.