Reorganise configuration files.

While working on documentation for the new wiki I noticed that the example config layout is extremely inconsistent. I have fixed this by making the following changes:

- Example config files (e.g. inspircd.conf.example) now end with .conf.example.
- Example text files (e.g. motd.txt.example) now end with .txt.example.
- Removed 'inspircd' from the name of multiple example files. This was pointless repetition as on system-wide installs we install into an 'inspircd' directory.
- Added missing oper motd example file.