Fix a minor style(9) issue.
Follow the ufs practice for disallowing permission changes as well as
writes to files for read-only file systems. Since there are already
checks in nandfs_setattr that return an error, this moves detection of
the error earlier.
Set the erase block size properly in the case the underlying media
doesn't advertise an erase block size.

Submitted by: bjg@
Pointy hat to: imp@
Add a new -K to suppress installing the kernel. Useful for creating
chroots that may never boot (such as build environments), or for
platforms that don't really need the kernel installed into the
filesystem.
Sort usage options.
Sort switch statements.