Consolidate dependency on OpenSSH headers.

[blunderer]

Create a new openssh/ folder and add a wrapper to OpenSSH includes. This allows the code to:

• Rely on includes.h from OpenSSH for better compatibility (see github.com/google/hiba/commit/642d64ffec5719178644078867bcd4ff88298621).
• Fix pre-processor conflicts on config.h symbols that are used in both projects.

This also remove uneeded includes in some source files.