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 e5c504c67c96f71dfdaa11f5ddbfa7cae21ce810)
(cherry picked from commit d12059e10ba46e1647292d753056a3da83181f9f)