* 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.cc

@@ -8,6 +8,8 @@ string nixDBPath = "/UNINIT";
 
 bool keepFailed = false;
 
+bool keepGoing = false;
+
 Verbosity buildVerbosity = lvlDebug;
 
 unsigned int maxBuildJobs = 1;