Mail: added support for XOAUTH2 and OAUTHBEARER authentication. This patch adds support for the OAUTHBEARER SASL mechanism as defined by RFC 7628, as well as pre-RFC XOAUTH2 SASL mechanism. For both mechanisms, the "Auth-User" header is set to the client identity obtained from the initial SASL response sent by the client, and the "Auth-Pass" header is set to the Bearer token itself. The auth server may return the "Auth-Error-SASL" header, which is passed to the client as an additional SASL challenge. It is expected to contain mechanism-specific error details, base64-encoded. After the client responds (with an empty SASL response for XAUTH2, or with "AQ==" dummy response for OAUTHBEARER), the error message from the "Auth-Status" header is sent. Based on a patch by Rob Mueller.