f3brew were working at sector level because
it was borrowing code from f3write/f3read which work at that level.
This patch writes new functions to fill out, and validate blocks.
This change is important because
f3probe needs to validate blocks as well for its coming features.
A side effect of this patch is that all experimental applications
(i.e. f3probe, f3brew, and f3fix) no longer depend on
code from f3write and f3read.
f3brew, f3 block read write, tests block devices
writing and reading blocks directly to devices.
This is a not-functional version of f3brew.
The main contribution of this patch is to reorganize libprobe.{h,c},
so f3brew, and any future application, can reuse the library.
