POP3_TYPE_ANY, or ~0, is written to pop3c->preftype in lib/pop3c.c, an
unsigned int variable.  The result of ~0 is -1, which caused a warning
due to writing a negative number to an unsigned variable.  To fix this,
make the expression ~0U so that its value is considered the unsigned
number UINT_MAX which is what SASL_AUTH_ANY does in curl_sasl.h.

Should a client application fail to decode an authentication message
received from a server, or not support any of the parameters given by
the server in the message, then the authentication phrase should be
cancelled gracefully by the client rather than simply terminating the
connection.

The authentication phrase should be cancelled by simply sending a '*'
to the server, in response to erroneous data being received, as per
RFC-3501, RFC-4954 and RFC-5034.

This patch adds the necessary state machine constants and appropriate
response handlers in order to add this functionality for the CRAM-MD5,
DIGEST-MD5 and NTLM authentication mechanisms.

Added the ability to use an XOAUTH2 bearer token [RFC6750] with POP3 for
authentication using RFC6749 "OAuth 2.0 Authorization Framework".

The bearer token is expected to be valid for the user specified in
conn->user. If CURLOPT_XOAUTH2_BEARER is defined and the connection has
an advertised auth mechanism of "XOAUTH2", the user and access token are
formatted as a base64 encoded string and sent to the server as
"AUTH XOAUTH2 <bearer token>".