Fix computation of .trunc register. Additionally, its value (and

Fix computation of .trunc register.  Additionally, its value (and
the value of the .ne register) is now always set before entering the
trap.

* src/roff/troff/div.cc (diversion::need): Set `truncated_space' and
`needed_space' before calling `space'.
(top_level_diversion::space): Remove special code for 'sp before the
first page.
Call `begin_page' with the discarded space as a parameter.
(top_level_diversion::begin_page): Add optional parameter to set
`truncated_space'.
* src/roff/troff/div.h: Updated.

* doc/groff.texinfo: Improve documentation of .sp, \n[.trunc], and
\n[.ne].

* tmac/an-old.tmac (SH, SS, TP, IP, HP, TS): Undo change 2002-10-26.
(LP): Remove superfluous call to `br'.
* tmac/doc-common (doc-paragraph): Undo change 2002-10-26.
* tmac/doc.tmac (Bd, Bl, doc-set-vertical-and-indent): Ditto.