cmd/5l, cmd/6l, cmd/8l: increase error buffer size

STRINGSZ (200) is fine for lines generated by things like
instruction dumps, but an error containing a couple file
names can easily exceed that, especially on Macs with
the ridiculous default $TMPDIR.

R=ken2
CC=golang-dev
https://golang.org/cl/11199043

src/cmd/5l/list.c

@@ -473,7 +473,7 @@ Oconv(Fmt *fp)
 void
 diag(char *fmt, ...)
 {
-	char buf[STRINGSZ], *tn, *sep;
+	char buf[1024], *tn, *sep;
 	va_list arg;
 
 	tn = "";

src/cmd/6l/list.c

@@ -423,7 +423,7 @@ Iconv(Fmt *fp)
 void
 diag(char *fmt, ...)
 {
-	char buf[STRINGSZ], *tn, *sep;
+	char buf[1024], *tn, *sep;
 	va_list arg;
 
 	tn = "";

src/cmd/8l/list.c

@@ -356,7 +356,7 @@ Iconv(Fmt *fp)
 void
 diag(char *fmt, ...)
 {
-	char buf[STRINGSZ], *tn, *sep;
+	char buf[1024], *tn, *sep;
 	va_list arg;
 
 	tn = "";