Accept -o for --data-offset

This was mentioned in the help but had been omitted.
Also accept flags with dashes

--data-offset didn't match the documentation.
Follow gnu style and accept dashes everywhere.
Correct typo in docs
udev: Skip floppies and cdroms
Take user-modified rules from /etc

Copy udev rules from /etc/udev/rules.d/ if they have been user-modified,
otherwise copy from /lib/udev/rules.d/ instead. Follows usual udev
conventions for modifying rules files.
Do not compile bcache-test by default

This way the Debian package can build without OpenSSL.