Apply cleaner rules for toolset initialization.

* tools/common.jam (check-init-parameters): New rule.
* tools/gcc.jam (init): Make use of the above.
* kernel/errors.jam (user-error): New rule.


[SVN r23084]

tools/build

@@ -1 +1 @@
-Subproject commit 6e8c4aa407d5ab5be38c191e48ce438f5009f2cb
+Subproject commit 1f8b5a619964d40fd9a1dbcaa9863be27e15d75f