makefs: sync option parsing with NetBSD

• add support for parsing different types; not just int
• homogenize option parsing
• fix single letter parsing
• remove duplicated code

NetBSD revisions:
cd9660.c 1.36 1.37 1.38 1.41 1.42 1.43
ffs.c 1.50 1.51 1.52 1.53 1.56 1.57
makefs.c 1.36 1.37 1.38 1.39 1.40 1.42 1.43 1.44 1.46
makefs.h 1.28 1.29 1.31 1.32

Obtained from: NetBSD
Sponsored by: The FreeBSD Foundation