mkfile: getopt() returns an int, not a char.

MFC after: 3 days
Reviewed by: kevans
Differential Revision: https://reviews.freebsd.org/D41804
Approved by: re (delphij)

(cherry picked from commit ef8abddf103d9dfd5660d50d15e6dbfbb2f47f62)
(cherry picked from commit 6bd2be4c4971f58c51509ce8856bd5331f979e85)