- created helper routine that will do authentication;
- routine tries each method in turn, first successful
one terminates it: makes things easier to read
and handle;
- renamed plainauth() inside offlineimap/imapserver.py
to loginauth(): the function does IMAP LOGIN authentication
and there is PLAIN SASL method, so previous name was
a bit misleading;
- slightly improved error reporting: all exceptions during
authentication will be reported at the end of the run;
- now loginauth() is never called if LOGINDISABLED is advertized
by the server; it used to be invoked unconditionally when
CRAM-MD5 fails, but we should respect server's opinion on
how to handle its users.
Signed-off-by: Eygene Ryabinkin <rea@codelabs.ru>