ren: end-of-sentence spacing when followed by font/size changes

The adj_nonl() command was added to prevent newlines for lines
containing only commands that do not print anything (like font or size
changes).  This patch makes sure render() does not call adj_nonl()
unnecessarily; otherwise, it may inhibit the insertion of a space
after end-of-sentence characters (when followed by a line break
starting with \f escape sequence).
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.
adj: keshideh (tatweel) adjustment

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