Fully separate authentication from IMAP access

Now imapsql module does not handle authentication. (it was not doing it so well
anyway)

sql_table module was introduced and used in the default configuration as
a replacement for functionality that was implemented by imapsql before.

Parts of maddyctl code were rewritten to make it work transparently with
any IMAP backend or credentials store.

Closes #212.

internal/module/auth.go

@@ -14,3 +14,13 @@ var (
 type PlainAuth interface {
 	AuthPlain(username, password string) error
 }
+
+// PlainUserDB is a local credentials store that can be managed using maddyctl
+// utility.
+type PlainUserDB interface {
+	PlainAuth
+	ListUsers() ([]string, error)
+	CreateUser(username, password string) error
+	SetUserPassword(username, password string) error
+	DeleteUser(username string) error
+}