Only advance the column position for printable characters, update manual

page to emphasise that we count column positions, not characters.

usr.bin/fold/fold.1

@@ -48,7 +48,7 @@ The
 .Nm
 utility is a filter which folds the contents of the specified files,
 or the standard input if no files are specified,
-breaking the lines to have a maximum of 80 characters.
+breaking the lines to have a maximum of 80 columns.
 .Pp
 The options are as follows:
 .Bl -tag -width indent
@@ -61,7 +61,7 @@ Fold line after the last blank character within the first
 .Ar width
 column positions (or bytes).
 .It Fl w Ar width
-Specify a line width to use instead of the default 80 characters.
+Specify a line width to use instead of the default 80 columns.
 .Ar Width
 should be a multiple of 8 if tabs are present, or the tabs should
 be expanded using

usr.bin/fold/fold.c

@@ -171,7 +171,8 @@ fold(width)
 				indx -= space;
 				col = 0;
 				for (i = 0; i < indx; i++)
-					col = newpos(col, buf[i]);
+					col = newpos(col,
+					    (unsigned char)buf[i]);
 			} else {
 				fwrite(buf, 1, indx, stdout);
 				col = indx = 0;
@@ -214,7 +215,8 @@ newpos(col, ch)
 			col = (col + 8) & ~7;
 			break;
 		default:
-			++col;
+			if (isprint(ch))
+				++col;
 			break;
 		}