The .ns and .rs requests are now honoured not only in the top-level

The .ns and .rs requests are now honoured not only in the top-level
but in all diversions (similar to UNIX troff).

This change is based on a patch from Tadziu Hoffmann
<hoffmann@usm.uni-muenchen.de>.

* src/roff/troff/div.h (diversion): Add `no_space_mode' member.
(top_level_diversion): Remove `no_space_mode' member.
* src/roff/troff/div.cc (diversion::diversion,
top_level_diversion::top_level_diversion): Updated.
(macro_diversion::output): Reset `no_space_mode'.
(no_space, restore_spacing): Use `curdiv' unconditionally.
(space_request, blank_line): Check `curdiv->no_space_mode'.
(no_space_mode_reg::get_value, no_space_mode_reg::get_string): Use
`curdiv' unconditionally.
* NEWS, src/roff/troff/troff.man, man/groff.man, doc/groff.texinfo:
Updated.