OpenBSD device length detection was returning the raw disk sector count from DIOCGPDINFO directly. That value is not bytes and it describes the physical/default disk label, which caused VeraCrypt to expose an incorrectly sized FUSE backing image through vnd for device-hosted volumes.
Use the current disklabel from DIOCGDINFO, derive the opened partition from the device minor number, and return the selected partition size in bytes. Keep the raw c partition on the whole-disk path by using DL_GETDSIZE there.
Also reject sector-misaligned device-hosted sizes during volume creation so new malformed OpenBSD device-hosted volumes are not created. Do not reject existing malformed headers at mount time, so users can still mount old OpenBSD-created volumes for recovery.
Refs #1589.
Refs #1593.
Allow normal file-hosted containers to use quick format in the Unix volume creation path by sizing the host file with ftruncate before backup headers are written.
Enable the GUI checkbox for normal file containers and honor --quick in text mode. Update the Unix HTML documentation for the weaker deniability properties of sparse or unwritten host regions.
We query the kern.geom.conftxt sysctl for the GEOM configuration to find
the partition offset. Technically speaking it would probably be better
to link against libgeom but this is less overall intrusive. Also
includes a small fix to find the parent device of an encrypted partition
when it is a GPT partition rather than a BSD slice.