#16 Regain registered nick after obtaining fall-back nick

When reconnecting while a stale bot process uses the registered nick, the fresh bot process gets assigned a fall-back nickname with an appended underscore.

Instead, the bot should regain its registered nick via NickServ. Replace proto.Recover, to be used when nick in use on nick change.

Relevant NickServ help output:

09:22 -NickServ- ***** NickServ Help *****
09:22 -NickServ- Help for REGAIN:
09:22 -NickServ-  
09:22 -NickServ- REGAIN regains access to your nickname from
09:22 -NickServ- a user that is using your nick.
09:22 -NickServ-  
09:22 -NickServ- If you are logged in to the account associated with
09:22 -NickServ- the nickname, you need not specify a password,
09:22 -NickServ- otherwise you have to.
09:22 -NickServ-  
09:22 -NickServ- Syntax: REGAIN <nick> [password]
09:22 -NickServ-  
09:22 -NickServ- Example:
09:22 -NickServ-     /msg NickServ REGAIN Dave2 goats
09:22 -NickServ- ***** End of Help *****

Seems to be related to 36e972ddf9 where I say 'REGAIN does not work'. I did not describe what happens if one tries and I can't remember. REGAIN should be re-investigated: it might have been a network-specific problem.