tbl: make unrecognized column mods less lethal

[g-branden-robinson]

src/cmd/tbl: add warn() function

Up to this point, tbl has had no nonfatal diagnostics. A forthcoming
change requires one.

src/cmd/tbl/t4.c: be robust to extensions

DWB 3.3 tbl and GNU tbl support an 'x' column modifier. (Heirloom
Doctools tbl and mandoc(1) also support it.) Plan 9 tbl aborts
processing of the entire input document if it encounters this characater
in a table description.

That's overkill; it's safe to ignore that modifier if it really is a
modifier, and the column descriptor has a classifier already, like 'L'
or 'N'. The sawchar local variable, of Boolean sense, in readspec()
appears to indicate that fact.

Many man pages in the wild, such as those of ncurses, use the 'x'
modifier; while this change doesn't make Plan 9 tbl implement the
extension, it does help Plan 9 render such pages on a best-effort basis.
Extending this toleration also permits man page authors to wangle a
portable hack into their pages by specifying an explicit width followed
by the 'x' modifier that implementations supporting it will honor
(example: 'Lw(22n)x').

The GNU tbl(1) man page says:

Column modifiers
Any number of modifiers can follow a column classifier. Modifier
arguments, where accepted, are case‐sensitive. If a given modifier
is applied to a classifier more than once, or if conflicting
modifiers are applied, only the last occurrence has effect. The
modifier x is mutually exclusive with e and w, but e is not
mutually exclusive with w; if these are used in combination,
x unsets both e and w, while either e or w overrides x.

Heirloom Doctools and mandoc(1) behave compatibly with the foregoing
description.

Before:

$ printf '.TS\nLx.\ntable cell\n.TE\n' | 9 tbl | 9 nroff -man | col | cat -s

Input:2: bad table specification character x
tbl quits

After:

$ printf '.TS\nLx.\ntable cell\n.TE\n' | 9 tbl | 9 nroff -man | col | cat -s

Input:2: warning: unrecognized column modifier character 'x'
     table cell

$ printf '.TS\nx.\ntable cell\n.TE\n' | 9 tbl | 9 nroff -man | col | cat -s

Input:2: bad table specification character x
tbl quits

$ printf '.TS\ntab(@);\nL Lx.\ncell one@cell two\n.TE\n' | 9 tbl | 9 nroff -man | col | cat -s

Input:3: warning: unrecognized column modifier character 'x'
     cell one   cell two

$ printf '.TS\ntab(@);\nL,x.\ncell one@cell two\n.TE\n' | 9 tbl | 9 nroff -man | col | cat -s

Input:3: bad table specification character x
tbl quits

I confess to using my own terminology rather than Mike Lesk's. See
https://man7.org/linux/man-pages/man1/tbl.1.html.