Implement new number register `.br' which is set to 1 if a macro has

Implement new number register `.br' which is set to 1 if a macro has
been called as .foo and to 0 if called as 'bar.  This is useful for
-mtrace so that requests can be reliably traced too.

* src/roff/troff/input.cpp (input_iterator): Add virtual function
`get_break_flag'.
(macro_iterator): Add `with_break' member and `get_break_flag'
function.
Update constructors.
(input_stack): Add function `get_break_flag'.
(break_flag_reg): New number register class.
(input_init_requests): Register `.br'.

* src/roff/troff/TODO: Updated.

* tmac/trace.tmac: s/!!sp/!!!sp/.
Modify definitions of `de', `de1', `am', and `am1' to use \n[.br].

* docs/groff.texinfo, man/groff.man, man/groff_diff.man, NEWS:
Document it.