While here:
1. Move FreeBSD-specific headers to a more centralized place, so that
it's easier to port mkimg.
2. Remove inclusion of <uuid.h> where we don't need it (copy-pasted).
3. Check the partition data given on the command line before we do any
I/O.
4. Add scheme_round() for rounding the partition size.
5. Have scheme_write call the scheme-specific write callback and have
it propagate errors back to the caller.
This single function takes a where argument to indicate the kind of
metadata to "size". This way we can also get rid of the "padding"
field in the scheme structure.
This should make it a little more understandable what's going on.
that need to be supported. Each scheme provides data and callbacks
for handling the scheme-specifics.
To this end, put each scheme in its own source file.
