veritysetup: add support for dm-verity options

veritysetup: add support for dm-verity options

This patch allows controlling the behaviour of dm-verity by reusing the
fifth argument that was used to set the roothash signature.

That argument is now a comma-separated list of dm-verity options in the
given format: option[=value]. The option is the name of the long option
in the world of veritysetup.

See `veritysetup(8)` for more details.

Note: The former ROOTHASHSIG the positional argument is now deprecated
in favour of the option root-hash-signature=(base64:SIG|FILE). However,
the previous format is maintained and a warning is logged.