\printlength{length} prints the value of a LaTeX length in the units
specified by \uselengthunit{unit} ('unit' may be any TeX length unit
except for scaled point, viz., any of: pt, pc, in, mm, cm, bp, dd or
cc). When the unit is pt, the printed length value will include any
stretch or shrink; otherwise these are not printed. The 'unit'
argument may also be PT, in which case length values will be printed
in point units but without any stretch or shrink values.
