unlock multiple passes and obsolete primary

* multiple passwords allowed in password file
* split on "\n", sideeffect: chop trailing slashes. fixes common mistake <(echo 'pass')
* remove accounts.Primary method
* do not fall back to primary account for mining

cmd/utils/flags.go

@@ -133,7 +133,7 @@ var (
 
 	UnlockedAccountFlag = cli.StringFlag{
 		Name:  "unlock",
-		Usage: "Unlock the account given until this program exits (prompts for password). '--unlock primary' unlocks the primary account",
+		Usage: "Unlock the account given until this program exits (prompts for password). '--unlock n' unlocks the n-th account in order or creation.",
 		Value: "",
 	}
 	PasswordFileFlag = cli.StringFlag{