add System.OsPath.getSearchPath

[joeyh]

On converting from System.FilePath to System.OsPath, I am missing getSearchPath.

I know, it's the only non-pure function in there. But complicating the transition to System.OsPath does not feel worth the benefit of making it all pure to me.

[hasufell]

I think the reason might be because we don't have a proper getEnv yet.

I laid the groundwork for that here haskell/win32#232

Now we have to implement getEnv and decide where it should live (is file-io appropriate?)