when preparing images for emulators or flash devices:
+ option '-C size' to create the underlying image file with given size.
Saves doing a 'dd' before, and especially it creates a sparse file
+ option '-@ offset' to build the FAT image at the specified offset
in the image file or device;
+ make the cluster size adaptive on the filesystem size.
Previously the default was 4k which is really unconvenient with
large media; now it goes from 512 bytes to 32k depending on
filesystem size (i still need to check whether it makes sense
to go further up, to 64k or above);
+ fix default geometry when not specified on the command line,
use 63 sectors/255 heads by default.
Also trim the size so it exactly a multiple of a track, to avoid
complaints in some filesystem code.
+ document all the above, plus some manual page clarifications.
MFC after: 4 weeks
b87d1601b6