Add \D't' from groff for line thickness

[Eolien55]

A new drawing primitive, mirroring groff's, was introduced: \D't'. It allows directly setting line width using units. \D't 1i' has expected results (a line width of an inch), whereas

.nr a 1i .dv set linewidth \na

does not, because the argument of linewidth is in thousandths of ems. It's also arguably more cumbersome.

This requires matching changes in neatpost.