Sync-to-go: src/pre-tbl..

Sync-to-go: src/pre-tbl..

Almost.
I took

- [8d036a3] (Improve `tbl' warnings., 2010-02-04)
  Implemented slightly different.
- [065f04d] (tbl: Add `nowarn' option., 2010-02-16).
  Implemented slightly different, and TODO we do set EXPAND_REG
  TODO to 0 even if nowarn is not set.
- [ef63f6a] (src/preproc/tbl/tbl.man: Mention trick to get
  abutting rules., 2011-07-12).
  For the being of everybody.
- [e508829] (src/preproc/tbl/tbl.man: Improve documentation of `_'
  and friends.)
  For the being of everybody.
- [ac4a8a8] (src/preproc/tbl/tbl.man: Better document `d' column
  specifier.)
  For the being of everybody.

I did not take

TODO - [4502c4a] (Document limitations with `#' characters, tbl,
       and eqn., 2009-10-28)
       See [3bc4a53] below.
TODO - [b690626] (Improve line numbering support in tbl and with
       me macros., 2011-07-20).
       That is a pretty large changeset that i has to be inspected.
       The good news is that e.tmac is by Eric Allman and thus of
       a quite free license (but anyway not covered by GPL3).
       Nonetheless it must be implemented in a running system.
TODO - [3bc4a53] ([tbl] Allow characters #, `, and ' as eqn
       delimiters.)
       And do add documentation.
TODO - [6360d7c] (Improve native Windows port., 2014-11-20,
       Eli Zaretskii).  File names should be treated in a library.