Update_bootcfg: Updated

Manual page.
Added some command-line options.
Dropped INHERIT stuff, following idea from David; now the
variables are read from /etc/boot.cfg.args, which is created
at setup time.
Make use of variables, which can be overriden to cater
with complex setup cases like /boot a separate partitions,
booting from another disk etc.
Update_bootcfg: allow to change minix_latest

Hability to remove a broken minix_latest,
and to fix it to point to the real latest still on disk.
Update_bootcfg: docs/UPDATING (*** edit date and merge with previous commit ***)