A VSH_OFLAG_REQ_OPT option means --optionname is required when used.
It will kill any ambiguity, even a !VSH_OFLAG_REQ option listed before
a VSH_OFLAG_REQ option, if the !VSH_OFLAG_REQ option is a
VSH_OFLAG_REQ_OPT option.

It will help us use optional argument with VSH_OT_ARGV argument.
Signed-off-by: NLai Jiangshan <laijs@cn.fujitsu.com>