grml_chroot: fix broken mount argument handling

Commit b4a4893b0b6 broke behavior of the grml-chroot script, as it then
passed all mount options as single argument, which then fails:

| + all_options_='-t proc none'
| + mount '-t proc none' /mnt/proc
| mount: /mnt/proc: can't find in /etc/fstab.
| + mountit sysfs sys
| [...]

Convert mount options into an array, to properly work also with quotes.