Special treatment for filesize 0b

[blindFS]

This PR is a partial fix for file size 0b which will lead to parse error in current grammar.

Since 0b is a special single terminal token in $.val_number and $.val_binary.
A perfect split into value 0 and unit b needs unnecessarily complicated tweaking of potential conflicts (also need to split 0b in other 2 rules).

With this PR however, 0b is added to $.val_filesize as a special case, no detailed fields generated:

  (pipeline [0, 0] - [1, 0]
    (pipe_element [0, 0] - [0, 2]
      (val_filesize [0, 0] - [0, 2])))

But it avoids the error with minimal effort.

[fdncred]

ya, that's probably a good strategy. thanks!