ed: Use the variable LINES for z

Some shells keep a variable LINES with the number of lines of the terminal
updated in every SIGWINCH. Using that variable makes easier to get a
full listing.

ed.1

@@ -1,4 +1,4 @@
-.Dd December 27, 2025
+.Dd December 31, 2025
 .Dt ED 1
 .Os sbase
 .Sh NAME
@@ -233,7 +233,10 @@ lines starting at the addressed line.
 If
 .Ar n
 is not specified then
-it is defaulted to 24.
+the value of $LINES - 1 is used.
+If the value of
+.Ar n
+is 0 or negative then the default value of 23 is used.
 .It ($)=
 Print the line number of the addressed line.
 The dot is unchanged.

ed.c

@@ -1270,6 +1270,7 @@ subst(int nth)
 static void
 docmd(void)
 {
+	char *var;
 	int cmd, c, line3, num, trunc;
 
 repeat:
@@ -1406,10 +1407,14 @@ repeat:
 	case 'z':
 		if (nlines > 1)
 			goto bad_address;
+
+		num = 0;
 		if (isdigit(back(input())))
 			num = getnum();
-		else
-			num = 24;
+		else if ((var = getenv("LINES")) != NULL)
+			num = atoi(var) - 1;
+		if (num <= 0)
+			num = 23;
 		chkprint(1);
 		deflines(curln, curln);
 		scroll(num);