Change ar's internal error routine to call err() instead of errx()

so that when ar has a problem (like trying to update a read-only
archive), you get a message like:

ar: xyzzy.a: Permission denined

instead of:

ar: xyzzy.a

Which doesn't really give the user any indication that the operation
really did fail.

This closes PR# 170.

usr.bin/ar/misc.c

@@ -142,5 +142,5 @@ error(name)
 	char *name;
 {
 
-	errx(1, "%s", name);
+	err(1, "%s", name);
 }