Added <minix/sys_config.h>. This file is intended to be included from
other, user-includable config files. It only defines names that don't
pollute the users namespace (start with _). <minix/config.h> still works
like always; it includes sys_config.h now and defines the 'messy' names
(such as CHIP) as the 'cleaner' names (such as _MINIX_CHIP).
Changed some of the other include files to use sys_config.h and the
'cleaner' names. This solves some (past and future) compilation problems.