Moving the sending of the final OK packet of the authentication process to
the client protocol plugin makes the authentication plugins simpler.
By reading the client's sequence and incrementing that by one, the client
protocol module will always send the correct sequence byte in the final OK
packet.
The MySQLCommon library contains functions used by both the protocol and
authenticator modules. The contents of the modutil.c file could also be
moved to this file if the functions in that file are only used by modules
and not the core.