Silence some unused variable warnings in tests/condition.[im]pure.lisp
Comments and rearrangements in {defboot,target-error}.lisp

Add and/or extend explanatory comments for the special
variables *HANDLER-CLUSTERS*, *RESTART-CLUSTERS*
and *CONDITION-RESTARTS*.

Slightly rearranged RESTART-NAME and RESTART-REPORT.
Make some RESTART slots read-only

The RESTART slots FUNCTION, REPORT-FUNCTION, INTERACTIVE-FUNCTION and
TEST-FUNCTION are never mutated and can thus be read-only.
Use new MAP-RESTARTS in FIND-RESTART, COMPUTE-RESTARTS; fix FIND-RESTART

* Both FIND-RESTART and COMPUTE-RESTARTS traverse, filter and select
  active restarts, potentially for a particular condition and
  potentially calling restart test functions. This common behavior has
  been factored into the new function MAP-RESTARTS.

* Remove *CONDITION-RESTARTS*; use new slot ASSOCIATED-CONDITIONS in
  RESTART structure instead.

* As stated in bug 774410, when given a RESTART instance, FIND-RESTART
  did not test whether the restart was still active and whether the
  restart had been associated to a different condition in the
  meantime. This behavior has been changed by calling MAP-RESTARTS which
  checks activity and association to conditions (sadly, making

    (find-restart RESTART-INSTANCE CONDITION-OBJECT)

  a bit slower than before).

* INVOKE-RESTART is also affected by the changes for bug
  774410. However, it does not respect restart test functions when
  called with a RESTART instance (This is underspecified in CLHS and
  other implementations behave in various ways. However, behaving
  differently would make some restarts un-invokable).

* As suggested in bug 769615, (find-restart SYMBOL ...) now calls
  MAP-RESTARTS instead of COMPUTE-RESTARTS, consing less and executing
  faster in many cases.

* New (find-restart :recheck-conditions-and-tests :bug-774410) test from
  Jean-Philippe Paradis from original bug report in
  tests/condition.impure.lisp checks the new behavior.

Fixes lp#769615, lp#774410