\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.
