[Groff] New register \n[.br] improved tracing support

[Werner LEMBERG]

>From the NEWS file:


o A new read-only number register `.br' is available which is set to 1
  if a macro is called as .foo and to 0 if called as 'foo.  This
  allows to reliably modify requests.

    .als address@hidden bp
    .de bp
    .  tm before bp
    .  ie \\n[.br] address@hidden
    .  el 'address@hidden
    .  tm after bp
    ..


  Enjoy!


     Werner