fmt: shrink word spaces with .ssh request

The new .ssh request specifies the amount (in percentage) by which
word spaces may be shrinked while filling.  The default value is zero.
fmt: specifying sentence space

This patch adds support for the second argument of .ss, as present
in Groff and Heirloom troff.
dir: support text direction with .>>, .<<, \> and \<

The following troff commands and requests specify text direction:

* .<< and .>> commands change the direction of text (.<< for r2l and .>>
  to l2r); the default direction is l2r.
* \< and \> change text direction temporarily

So for instance one can write:

  .<<
  This is a paragraph in a right-to-left language
  with some embedded \>left-to-right words\< in it.
  .>>

Normal and temporary indentation indent the right side of the lines
after ".<<"; to adjust the left side, this patch introduces base
indentation \n(.I, which can be specified as the second argument of
".in" command.  Base indentation always indents the left side of the
lines, regardless of the current text direction and ".ti" requests.
Note that the sum of \n(.i (or the temporary indent, if specified),
\n(.I, and the length of adjusted lines equals \n(.l.
fmt: keshideh justification

With ".ad k" keshideh characters are inserted in output words,
before adjusting with spaces.