* A flag `--keep-going / -k' to keep building goals if one fails, as

much as possible.  (This is similar to GNU Make's `-k' flag.)

* Refactoring to implement this: previously we just bombed out when
  a build failed, but now we have to clean up.  In particular this
  means that goals must be freed quickly --- they shouldn't hang
  around until the worker exits.  So the worker now maintains weak
  pointers in order not to prevent garbage collection.

* Documented the `-k' and `-j' flags.

src/libstore/globals.hh

@@ -29,6 +29,10 @@ extern string nixDBPath;
 /* Whether to keep temporary directories of failed builds. */
 extern bool keepFailed;
 
+/* Whether to keep building subgoals when a sibling (another subgoal
+   of the same goal) fails. */
+extern bool keepGoing;
+
 /* Verbosity level for build output. */
 extern Verbosity buildVerbosity;