- helper support (mount.type).
- helpers need to be in $PATH, if needed we can add a check for
/sbin/mount.XXXX
- pass -B, -M, -R to helper, its more reliable to pass these named
options with -o however.
- allow prefix "no" for which type no action should be taken:
mount -a -t nonfs,ext4
fix bugs:
- dont modify me->mnt_opts (used strtok).