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

cmd/zfs/zfs_main.c

@@ -2239,7 +2239,7 @@ zfs_do_upgrade(int argc, char **argv)
 	boolean_t showversions = B_FALSE;
 	int ret = 0;
 	upgrade_cbdata_t cb = { 0 };
-	signed char c;
+	int c;
 	int flags = ZFS_ITER_ARGS_CAN_BE_PATHS;
 
 	/* check options */
@@ -3933,7 +3933,7 @@ static int
 zfs_do_snapshot(int argc, char **argv)
 {
 	int ret = 0;
-	signed char c;
+	int c;
 	nvlist_t *props;
 	snap_cbdata_t sd = { 0 };
 	boolean_t multiple_snaps = B_FALSE;

cmd/zpool/zpool_main.c

@@ -973,7 +973,7 @@ zpool_do_remove(int argc, char **argv)
 	int i, ret = 0;
 	zpool_handle_t *zhp = NULL;
 	boolean_t stop = B_FALSE;
-	char c;
+	int c;
 	boolean_t noop = B_FALSE;
 	boolean_t parsable = B_FALSE;
 

cmd/zstreamdump/zstreamdump.c

@@ -237,7 +237,7 @@ main(int argc, char *argv[])
 	struct drr_write_embedded *drrwe = &thedrr.drr_u.drr_write_embedded;
 	struct drr_object_range *drror = &thedrr.drr_u.drr_object_range;
 	struct drr_checksum *drrc = &thedrr.drr_u.drr_checksum;
-	char c;
+	int c;
 	boolean_t verbose = B_FALSE;
 	boolean_t very_verbose = B_FALSE;
 	boolean_t first = B_TRUE;