chkuser 2.0 patch design

Main files

The whole of chkuser code is contained within separated files:

chkuser.h	contains base definitions for chkuser
chkuser.c	contains all the code necessary for chkuser
chkuser_settings.h	contains all the settings necessary to customize chkuser behaviours

Link within qmail-smtpd is done editing only some hook lines within qmail-smtpd.c, and modifying some lines in Makefile and conf-cc.

Following a semplified flow regarding chkuser design.