Fix \$@ and \$* to handle any number of arguments.
* src/roff/troff/input.h (BEGIN_QUOTE, END_QUOTE): New special
characters.
* src/roff/troff/input.cpp (input_iterator::internal_level):
Removed.
(input_stack): New member functions `increase_level' and
`decrease_level'.
(input_stack::get_level): Don't use `internal_level'.
(get_copy, token::next): Handle BEGIN_QUOTE and END_QUOTE.
(end_quote_iterator): Completely removed.
(interpolate_arg): Build string for \$@ and \$* which is then
pushed onto the input stack.