Use different nicknames if they are not available
Irccd will silently fail to connect if nickname is already used. Do like many other IRC clients by testing different nicknames if they are already used.
- Option identity.nickname should be a list
- Patch libircclient to detect already used nickname
Irccd: use system username, closes #562
Use the system user login name as both nickname and username to avoid
having collision with possible existing irccd.
This is currently only possible if the system has `getlogin` function
which targets primarily unix platforms. Windows support will be added