Begin cleanup of scaling units.

Begin cleanup of scaling units.
Note that we use 240u := 1i for all devices, even -Tps and -Tpdf.
Big fix of -Tascii rendering of f, m, and u.
Small fix of -Tascii rendering of c.
Big fix of -Thtml rendering of u.
Big fix of -Tps rendering of m, p, and u.
Clarify -Tps rendering of c.
Correct documentation of scaling units, in particular with respect to u.
This for example improves rendering of the OpenGL manuals.
Joint work with kristaps@.