makefs: eliminate global cd9660 structure

For diff reduction with NetBSD

NetBSD file versions:
cd9660.c 1.39
cd9660.h 1.19
cd9660/cd9660_debug.c 1.12
cd9660/cd9660_eltorito.c 1.20
cd9660/cd9660_write.c 1.16
cd9660/iso9660_rrip.c 1.12
cd9660/iso9660_rrip.h 1.6

Reviewed by: ngie
Obtained from: NetBSD
MFC after: 1 month
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D9627