fstype checking, and back-end "ffs" support.
- Consistently use at least uint32_t for blocknumbers (vs. int / long / ...)
- Consistently use uintXX_t instead of u_intXX_t.
- Move various duplicated prototypes into "installboot.h"
existing backends, but future ones need it)
- use "bootstrap" instead of "boot block" in various messages where appropriate
- rename some members of ib_params
to use when they don't support a particular method
- don't automatically generate machines.c anymore
- add entries for i386 (temporarily unsupported) and shark
(permanently unsupported)
