handle AUTHINFO USER & AUTHINFO PASS as defined in RFC

[sanderjo]

Description

Closes #673

So: AUTHINFO USER / PASS sequence handling according to RFC4643.

Lib changes

None

Testing

Old:

[INFO] [10] Incoming connection from: 127.0.0.1
[DETAIL] [10] Received: authinfo user myusername123
[DETAIL] [10] Received: QUIT
[DETAIL] [10] Closing connection

New:

[INFO] [3] Incoming connection from: 127.0.0.1
[DETAIL] [3] Received: authinfo user myusername123
[DETAIL] [3] Received: authinfo pass mypassword456
[DETAIL] [3] Received: QUIT
[DETAIL] [3] Closing connection