Factor out common code used to underline/strike out math

Factor out common code used to underline/strike out math

Also use the exact amount of vertical space TeX adds after a
math display (instead of 1ex) for vertically shifting the box.
We have to use \belowdisplayshortskip here, instead of
\belowdisplayskip, because the math formula is typeset by alone
in a box, and thus there is no following line.