Fix incompatibility between `.de1' and `.do'. Without this change,

Fix incompatibility between `.de1' and `.do'.  Without this change,
the following snippet

  .de1 xx
  .  tm \\n(.C
  ..
  .cp 1
  .do xx

prints 1 instead of 0.

* src/roff/troff/input.cc (do_request): If a macro gets processed,
call tok.next().
(interpolate_macro): Add optional argument.  Update callers.
(request::invoke): Add optional argument.
(macro::invoke): Add optional argument to delay call of tok.next().

* src/roff/troff/request.h (request_or_macro): Add argument to
`invoke' member.  Update all derived classes.

* doc/groff.texinfo: Improve documentation of .do request.