IMAP authentication improvements

Changes:

Implement the PLAIN SASL mechanism.  IMAPv4rev1 assures its availability
so long as the connection is encrypted.  The big advantage of PLAIN over
IMAP "LOGIN" is that PLAIN uses UTF-8 encoding for the user name and
password, whereas "LOGIN" is only safe for 7-bit US-ASCII -- the encoding
of 8-bit data is undefined.

(Note that RFC 6855 says that IMAP "LOGIN" does not support UTF-8, and
clients must use IMAP "AUTHENTICATE" to pass UTF-8 user names and
passwords.)

Honor the "LOGINDISABLED" CAPABILITY (RFC 2595) when the server declares
it.  There's no sense transmitting a password in the clear when it is
known that it will be rejected.

No attempt is made to try CRAM-MD5 if the server doesn't profess to
support it in its CAPABILITY response. (This is the same behavior as
Thunderbird.)

Extract code from ImapConnection.open into new method
ImapConnection.login.

Extract code from ImapConnection.executeSimpleCommand into new method
ImapConnection.readStatusResponse.

Related issues:  6015, 6016