LU-13701 utils: fix usage of getopt_long in portals

LU-13701 utils: fix usage of getopt_long in portals

Due to char being unsigned by default, using this type to store
the int return value of getopt_long causes parsing issues on arm
platform, seen with lctl net_drop_add options. Fix it.

Test-Parameters: trivial testgroup=review-ldiskfs-arm
Signed-off-by: Serguei Smirnov <ssmirnov@whamcloud.com>
Change-Id: Ibef6ba347a90d9ed3cd4bd9346ed9f79f0120b87
Reviewed-on: https://review.whamcloud.com/40036
Tested-by: jenkins <devops@whamcloud.com>
Reviewed-by: James Simmons <jsimmons@infradead.org>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Chris Horn <chris.horn@hpe.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>