privileged NS INFO should show whether a name was previously unregistered

[slingamn]

reported by @Mikaela:

1. NS INFO on an unregistered (as oppose to erased) account reports that the account does not exist, or words to that effect
2. NS REGISTER and NS SAREGISTER report that the name cannot be re-registered since it already exists, or words to that effect

Desired behavior:

1. For a privileged operator, NS INFO should report that the name was previously unregistered
2. NS REGISTER should report "name is reserved" as opposed to "already exists"

[slingamn]

This is running into the following misfiring check:

ergo/irc/accounts.go

Lines 434 to 437 in 0309276

	// can't register an account with the same name as a registered nick
	if am.NickToAccount(account) != "" {
	return errAccountAlreadyRegistered
	}

which was originally intended to stop people from registering a nickname that someone else had already GROUP'ed.