make(1): remove Dir_InitDir

The function name had been too ambiguous since it didn't mention the
particular directory that was initialized.  Instead of that function,
Dir_InitCur is called directly from main_Init.

The pseudo CachedDir entry ".DOTLAST" is initialized at the very
beginning.  The observable behavior is unchanged since this a
memory-only object with no connection to the file system.

(rillig)