Add support for uploading SSH keys to a server.

This introduces the abliity to upload an existing id_dsa or id_rsa key
to the server. The key will be validated and then stored as id_dsa or id_rsa,
depending on the type (and regardless of the original filename).

This allows an administrator to easily provide a known good key instead of
having to copy the file over as root.

Tested by uploading different id_dsa and id_rsa files, with different names.
All the actual keys were uploaded with the proper names. Everything else returned
an error.

Tested that generating keys still worked fine.