init: simplify keyword_map

I've heard that keyword_map is too complex, in particular the tuple
and the pair in BuiltinFunctionMap, so this change removes a lot of
that complexity and, more importantly, better documents how all of
this works.

Test: boot, init unit tests

Change-Id: I74e5f9de7f2ec524cb6127bb9da2956b5f307f56
diff --git a/init/subcontext.h b/init/subcontext.h
index 16bd870..591521f 100644
--- a/init/subcontext.h
+++ b/init/subcontext.h
@@ -60,7 +60,7 @@
     android::base::unique_fd socket_;
 };
 
-int SubcontextMain(int argc, char** argv, const KeywordFunctionMap* function_map);
+int SubcontextMain(int argc, char** argv, const BuiltinFunctionMap* function_map);
 std::vector<Subcontext>* InitializeSubcontexts();
 bool SubcontextChildReap(pid_t pid);
 void SubcontextTerminate();