Fixed compiler and valgrind warnings

Added missing DBUG_xxx_RETURN statements
Fixed some usage of not initialized variables (as found by valgrind)
Ensure that we don't remove locked tables used as name locks from open table cache until unlock_table_names() are called.
This was fixed by having drop_locked_name() returning any table used as a name lock so that we can free it in unlock_table_names()
This will allow Tomas to continue with his work to use namelocks to syncronize things.

Note: valgrind still produces a lot of warnings about using not initialized code and shows memory loss errors when running the ndb tests

mysql-test/valgrind.supp

@@ -153,6 +153,15 @@
    fun:gzflush
 }
 
+{
+   libz deflate3
+   Memcheck:Cond
+   obj:*/libz.so.*
+   obj:*/libz.so.*
+   fun:deflate
+   fun:do_flush
+}
+
 #
 # Warning from my_thread_init becasue mysqld dies before kill thread exists
 #