Merge extras/m_privdeaf into m_deaf and update documentation.

Merge extras/m_privdeaf into m_deaf and update documentation.

- Merge the 2.0 extras module m_privdeaf (usermode +D for deaf to
user messages and notices) as they have a similar purpose.
- Improve the channel deaf logic where a known case of bypassing
still looped the channel userlist building an empty exemption list.
- Improve the comments within the code.
- Update the documentation with the previously undocumented
configuration to deaf and the new privdeaf configuration.