OpenZFS 7742 - zfs send wrong error message with invalid long opts

There are two cases:

    1. if an invalid flag is passed, and
    2. if a valid flag is not given a parameter.

In the case of (1), the flag is either short or long. For short flags,
optopt contains the character of the flag. For long, it contains zero,
and we can access the long flag using argv and optind.

In the case of (2), if the flag is short, optopt contains the character
of the flag. If the flag is long, the value in the 4th column of the
long_options table, for that flag, is returned.

We could case over all those values, or we could simply use argv and
optind again.

Note that in the case of something like `--resume`, which is also `-t`,
"t" will be returned if an argument is not provided; so the error
message will say `'t': argument not provided` or similar. This could be
fixed by making it so long and short options don't use the same
character flag, and then combining them in the switch/case statement,
but I didn't think the ugliness of the code would be worth the small
usability enhancement.

Authored by: Paul Dagnelie <[email protected]>
Reviewed by: Dan Kimmel <[email protected]>
Reviewed by: Steve Gonczi <[email protected]>
Reviewed by: Robert Mustacchi <[email protected]>
Reviewed-by: Brian Behlendorf <[email protected]>
Ported-by: George Melikov <[email protected]>

OpenZFS-issue: https://www.illumos.org/issues/7742
OpenZFS-commit: https://github.com/openzfs/openzfs/commit/6d69b40
Closes #5702 

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