fmt: specify the minimum length of paragraph last lines with .pmll

The request .pmll sets the minimum length of the last line of
formatted paragraphs, specified as a percentage of \n(.l, when
formatting paragraphs with .ad p.
fmt: consider the number of spaces while filling paragraphs

This updates the function for calculating the cost of breaking words
into lines to consider the number of spaces in each line.  The main
motivation is that a line with many spaces can be stretched much
better than a line with few large words.

This also updates the meaning of the argument of .hyp request: Now
useful values are between 0 and probably 1000; an argument of 100
assigns to each hyphenation the cost of stretching the spaces of the
line one hundred percent.  The default value is zero.