Add US-english hyphenation exceptions (converted from Barbara

Add US-english hyphenation exceptions (converted from Barbara
Beeton's hyphenation exception log reports which appear irregularly
in TUGBoat).

* tmac/hyphen.us: Updated to latest version.
* tmac/README: Updated.
* tmac/hyphenex.us, tmac/hyphenex.sh: New files.
* tmac/troffrc: Load `hyphenex.us'.
* tmac/Makefile.sub (NORMALFILES): Add `hyphenex.us'.
* doc/groff.texinfo: Updated.

Improve hyphenation slightly.  This is a first step in redesigning
the hyphenation algorithm to make it more flexible (e.g. allowing
kerns and ligatures between the hyphenation character and the
following character -- while not used normally in English, other
languages like German would benefit).

* src/roff/troff/env.cc (environment::hyphenate_line): Use
assertion instead of if-clause.
Let `get_hyphen_list' return the number of involved characters in
the hyphenation pattern instead of computing it directly (which
often yields too small values).
* src/roff/troff/node.h (*::get_hyphen_list): Add second parameter.
* src/roff/troff/node.cc (*::get_hyphen_list): Handle new second
parameter.

Fixing a bug which caused groff to hang if the hyphenation exception
dictionary tried to grow.

* src/roff/troff/env.cc (hyphen_trie::insert_hyphenation,
hyphen_trie::read_patterns_file, do_hyphenation_patterns_file): Use
pointer to dictionary.