This allows modules to be used as a table to check account existence.
auth/pass_table: Implement module.Table interface
auth/shadow: Implement module.Table interface
auth/plain_separate: Implement module.Table interface
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.
