Possible fix for UNAVAILABLE handling

Hi, I've been having issues with IDLE not working properly and getmail
randomly closing every x hours. I traced it back to an occasional
negative answer to AUTHENTICATE, but it seems getmail is treating the
case as a permanent error, like an unsupported authentication scheme,
while in my case I think it's something related to server routine
maintenance.

The errors are usually something like:

credential/login error ([UNAVAILABLE] Account is temporarily unavailable.)
credential/login error ([UNAVAILABLE] Internal error occurred. Refer to
server log for more information.)

Now, RFC 5530 says UNAVAILABLE is a "Temporary failure because a
subsystem is down", so I propose the following change:

In _retrieverbases.py:

def initialize(self, options):
[...]
            except imaplib.IMAP4.error, o:
+               if o.startswith('[UNAVAILABLE]'):
+                   raise getmailLoginRefusedError(o)
                raise getmailCredentialError(o)

In getmail:

def go(configs, idle):
[...]
        except getmailCredentialError, o:
            errorexit = True
            retriever.abort()
            log.error('%s: credential/login error (%s)\n' % (configfile, o))
            if options['logfile']:
                options['logfile'].write('credential/login error (%s)' % o)

+       except getmailLoginRefusedError, o:
+           retriever.abort()
+           log.error('%s: login refused error (%s)\n' % (configfile, o))
+           if options['logfile']:
+               options['logfile'].write('login refused error (%s)' % o)

So, we use getmailLoginRefusedError ("Error raised when the server is
just refusing logins due to reasons other than credential problems (when
that is possible to detect):  server too busy, service shutting down,
etc.") to avoid exiting from the idle loop.

My two cents, btw (I opened getmail source code today for the first time).

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