bio: add Bfdopen() from plan9port
xen: fix mtrr dummy functions
kernel: get rid of /boot/boot parametrization

there is no use for "bootdisk" variable parametrization
of /boot/boot and no point for the boot section with its
boot methods in the kernel configuration anymore. so
mkboot and boot$CONF.out are gone.

move the rules for bootfs.paq creation in 9/boot/bootmkfile.
location of bootfs.proto is now in 9/boot/bootfs.proto.
our /boot/boot target is now just "boot".
png: fail on invalid bpc
kernel: remove *.acid files in nuke target instead of $CONF.clean target
merge