mount_cd9660 segfaults

["Simon 'corecode' Schubert" <corecode@xxxxxxxxxxxx>]

mount_cd9660 segfaults if you don't specify -L.

The code seems to be copied from mount_msdos, but FreeBSD has a 
different code and argument in mount_cd9660.  We should try to stay 
close to FreeBSDs behavior.

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