secmodel_extensions: Split vfs part into separate .c file.

This way we can provide weak rumpkern stubs that don't require
rumpvfs for things that are relevant to vfs, but if you do link
rumpvfs then

secmodel_extensions: Split vfs part into separate .c file.

This way we can provide weak rumpkern stubs that don't require
rumpvfs for things that are relevant to vfs, but if you do link
rumpvfs then you get the same logic in secmodel extensions.

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