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

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

Author: g-branden-robinson

src/cmd/tbl/t4.c

@@ -47,8 +47,18 @@ readspec(void)
 		default:
 			if (c != tab) {
 				char buf[64];
-				sprint(buf, "bad table specification character %c", c);
-				error(buf);
+				if (sawchar)
+					if (c != '\'') {
+						sprint(buf, "unrecognized column modifier character '%c'", c);
+						warn(buf);
+					} else {
+						sprint(buf, "unrecognized column modifier character \"%c\"", c);
+						warn(buf);
+					}
+				else {
+					sprint(buf, "bad table specification character %c", c);
+					error(buf);
+				}
 			}
 		case ' ': /* note this is also case tab */
 			continue;