veritysetup: print help for --help/-h/help

In general our commands print help on --help, but here this would trigger
the error that two arguments are needed. Let's make this more user-friendly.

(cherry picked from commit 5d5e43cc)
(cherry picked from commit 8d9471cbca46115e6411b78abc5bc67390940cf1)
(cherry picked from commit e5715ab1c75a1b80c1a9a30f7d353b5fd09a93f2a)