mkfs: Take two block devices as arguments. Write everything to metadata
dev, and the superblock to the data dev. UUIDs match. Differentiate by
checking a bit in a new "flags" field in the superblock.
Refactor device_size() a little. Convert spaces to tabs.
Move code to pretty-print sizes to dev.c so we can use it in error
messages there, as well as in mkfs.c.
print: Include flags in output.
Add -D and -M options for setting max dev sizes
Allow sizes to be specified using units like "K", "G" etc.
Note: -D option replaces -S option, and uses above units rather than
the number of 4k data blocks.
Update man pages for cmdline changes.
Signed-off-by: Andy Grover <agrover@versity.com>
