Added the `return' request to end a macro immediately. It simply

Added the `return' request to end a macro immediately.  It simply
pops iterators from the input stack until a macro iterator is found.

* src/roff/troff/input.cc (input_iterator::is_macro,
macro_iterator::is_macro): New member.
(input_return_boundary): New class to signal an immediate return
to while_request().
(input_stack::add_return_boundary, input_stack::is_return_boundary):
New functions.
(input_stack::clear): Use it.
(input_stack::pop_macro): New function.
(while_request): Use `is_return_boundary()'.
(return_macro_request): New function.
(init_input_requests): Use it.

* src/roff/troff/TODO: Updated.
* NEWS, src/roff/troff/troff.man, man/groff.man: Document it.

* src/roff/troff/input.cc (input_iterator::is_boundary): Minor
cleanup.