makefs: remove vestigial '?' cases from top-level getopt(3) loop

getopt(3) returns '?' when it encounters a flag not present in the in
the optstring or if a flag is missing its option argument. We can
handle this case with the "default" failure case with no loss of
legibility.

Obtained from: OpenBSD makefs.c 1.22

(cherry picked from commit cc4adc4c4e473d47b687d2ac2757157abd728c81)