Add support for the following installpkg flags:

```
--warn (warn if files will be overwritten, but do not install)
--md5sum (record the package's md5sum in the metadata file)
--no-overwrite When extracting the package, do not overwrite
  existing files. Usually, this option should not be used.
  It exists so that upgradepkg can use it for the second
  installation pass. The first pass has already overwritten
  the previous package's files, and this will catch the few
  corner cases without generating unnecessary writes.
```