PC kernel

The PC kernel is currently being updated to bring it up-to-date with
Plan 9 support (the original Inferno PC kernel was derived from an
extended subset of an earlier edition of Plan 9).  In particular,
although it can be configured as a server, graphics support is
still being provided.  (The kernel level code in vga*.c, devvga.c and screen.c
is probably near the final version, but some Limbo code is needed to
set the right graphics modes.)
