sasl: Moved ntlm authentication message handling from smtp.c

Moved the ntlm message creation and decoding from smtp.c into the sasl
module to allow for use by other modules such as pop3.

sasl: Moved login authentication message creation from smtp.c

Moved the login message creation from smtp.c into the sasl module
to allow for use by other modules such as pop3.

sasl: Moved plain text authentication message creation from smtp.c

Moved the plain text message creation from smtp.c into the sasl module
to allow for use by other modules such as pop3.