Endless loop in zpool_do_remove() on platforms with unsigned char

On systems where "char" is an unsigned type the value returned by
getopt() will never be negative (-1), leading to an endless loop:
this issue prevents both 'zpool remove' and 'zstreamdump' for
working on some systems.

Reviewed-by: Igor Kozhukhov <igor@dilos.org>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Chris Dunlop <chris@onthe.net.au>
Signed-off-by: loli10K <ezomori.nozomu@gmail.com>
Closes #8789