Rewrite TmpFS class definition.
Rewrite TmpFS class definition.

Principle changes are:

* Abstract calculating the format for the size mount option into
_sizeOption method.

* Substitute FS.mountopts for TmpFS._options instance variable.

* Whether mounting or remounting always use default options if no
special options set and always use size mount option if required.

* do not set size to what the tmpfs default usually is. When it is
necessary to get the actual size, TmpFS._getExistingSize() will be used.

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