cd9660: Apply the file and directory mode masks bits to all 12 access bits

This permits the mask bits to control the upper 3 bits used for setuid,
setgid, and sticky permissions. While here, clarify the manpage language
as non-Rockridge volumes with extended attributes can also supply users
and groups along with permissions.

Reviewed by: olce
Fixes: 82f2275b73e5 cd9660: Add support for mask,dirmask,uid,gid options
Differential Revision: https://reviews.freebsd.org/D47357