test/zdtm.sh: move -h parsing to inside case
test/zdtm.sh: move -h parsing to inside case

It's cleaner to have all options parsed in the same way (and in the same
manner).

Factor out usage() and call it when -h is found.

Also, print usage text to stdout not stderr.

Signed-off-by: Kir Kolyshkin <c0159612b0ef32139322233232df1a0d189c4b9b@openvz.org>
Acked-by: Andrew Vagin <74c05afa918858348d6db8037d82fa8124870113@parallels.com>
Signed-off-by: Pavel Emelyanov <c9a32589e048e044184536f7ac71ef92fe82df3e@parallels.com>

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