`cellspacing` attribute per HTML5 § 14.3.9.
Table layout code has been refactored to push the spacing down to
rowgroups and rows; this will aid the implementation of
`border-collapse` as well.
This commit also fixes two nasty issues in table layout:
* In fixed layout, extra space would not be divided among columns that
had auto width but had nonzero minimum width.
* In automatic layout, extra space would be distributed to constrained
columns as well even if unconstrained columns with percentage equal to
zero were present.
