zhack: fix getopt return type

This fixes zhack's command processing on ARM. On ARM char
is unsigned, and so, in promotion to an int, it will never
compare equal to -1.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Nathaniel Wesley Filardo <nwf@cs.jhu.edu>
Closes #7016