restart-case: Spurious unused variable warnings.

restart-case without any restart clauses used to signal an unused
variable warning, add ignorable declaration.
Fixes lp#1113859
mention :TIME in *SAMPLING-MODE* docstring
make SB-SPROF:WITH-PROFILING not loop by default

 ...and in non-loop mode it properly returns the result values of the
 macro body.
grab-bag of SB-SPROF improvements.

 (1) Thread distribution handler must not check *SAMPLING*, as it
     is thread-local.

 (2) Return the values from the final round when looping.

 (3) Better warning on no sampling process: list the reasons users can
     do something about.

 (4) Update WITH-PROFILING docstring to reflect reality.

 (5) When printing *SAMPLES*, don't print the array.