veritysetup-generator: add support for veritytab

This adds the support for veritytab.

The veritytab file contains at most five fields, the first four are
mandatory, the last one is optional:
 - The first field contains the name of the resulting verity volume; its
   block device is set up /dev/mapper/</filename>.
 - The second field contains a path to the underlying block data device,
   or a specification of a block device via UUID= followed by the UUID.
 - The third field contains a path to the underlying block hash device,
   or a specification of a block device via UUID= followed by the UUID.
 - The fourth field is the roothash in hexadecimal.
 - The fifth field, if present, is a comma-delimited list of options.
   The following options are recognized only: ignore-corruption,
   restart-on-corruption, panic-on-corruption, ignore-zero-blocks,
   check-at-most-once and root-hash-signature. The others options will
   be implemented later.

Also, this adds support for the new kernel verity command line boolean
option "veritytab" which enables the read for veritytab, and the new
environment variable SYSTEMD_VERITYTAB which sets the path to the file
veritytab to read.