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 setting these variables in
from correct REPL buffer.

* contrib/slime-repl.el (slime-repl-eval-string): slime-eval
SWANK:SET-REPL-VARIABLES when SWANK acknowledges with OK.

* contrib/swank-repl.lisp (*last-repl-form*, *last-repl-values*): new
variables.
(*repl-set-variables-function*): new variable.
(set-repl-variables): new function, extracted from REPL-EVAL,
switches to *REPL-SET-VARIABLES-FUNCTION* if that is non-nil.
(repl-eval): don't set auto variables here, defer that for later.