Made FUNCTIONs work in insert and select queries, as well as nested function invocations.

Had to add a cahing mechanism which is in parts an ugly kludge, but it will be
reworked once the real SP caching is implemented.

sql/sql_class.h

@@ -553,6 +553,7 @@ public:
   bool       prepare_command;
   bool	     tmp_table_used;
   sp_rcontext *spcont;		// SP runtime context
+  List<sp_head> spfuns;		// SP FUNCTIONs
 
   /*
     If we do a purge of binary logs, log index info of the threads