ren: do not use adj struct to implement .tl

This is the first of a series of simplifying patches to adj.c.
adj: access n_ll and n_i directly
adj: store the words as strings
reg: \n[.neat] is always one!
fmt: collect words in fmt buffer for whole paragraphs

This patch prepares neatroff for filling whole paragraphs
at once.
fmt: filling whole paragraphs without hyphenating
fmt: new hyphenation support with penalties

The .hyp request can specify the hyphenation penalty.  An argument of
1000 assigns the cost of formatting an empty line to each hyphenation;
a value between 0 and 100 seems feasible for common line lengths.
fmt: not hyphenating last lines if requested
ren: allow right-adjusted un-filled text

It seems an inconsistency in the original troff that the
combination of ".nf" and ".ad r" or "ad c" does not right-adjust
or centre the unfilled input lines.

This is effective only when compatibility mode is off.
fmt: preserve current font and size when hyphenating