gdb: Add an error when 'list -' reaches the start of a file.

When a a user uses 'list +' to list forward through a source file they
eventually reach the end of the source file.  Subsequent uses of 'list
+' result in an error message like this, that let the user know they are
at the end of the source file:

  Line number XXX out of range; FILENAME has YYY lines.

Compare this to the current behaviour of 'list -' which lists backwards
through a source file.  When the user reaches the beginning of the
source file, subsequent uses of 'list -' result in the command silently
returning.  This can be confusing if the previous uses of 'list -' have
scrolled off the users display, the user receives no reminder that the
have already seen the start of the file.

After this commit a use of 'list -' when the user has already seen the
start of a file will receive the following error:

   Already at the start of FILENAME.

gdb/ChangeLog:

	* cli/cli-cmds.c (list_command): Add an error when trying to use
	'-' to scan read off the start of the source file.

gdb/testsuite/ChangeLog:

	* gdb.base/list.exp (test_list_forward): Add end of file error
	test.
	(test_repeat_list_command): Add end of file error test.
	(test_list_backwards): Add beginning of file error test.

gdb/cli/cli-cmds.c

@@ -940,10 +940,15 @@ list_command (char *arg, int from_tty)
       /* "l -" lists previous ten lines, the ones before the ten just
 	 listed.  */
       else if (arg[0] == '-')
-	print_source_lines (cursal.symtab,
-			    max (get_first_line_listed ()
-				 - get_lines_to_list (), 1),
-			    get_first_line_listed (), 0);
+	{
+	  if (get_first_line_listed () == 1)
+	    error (_("Already at the start of %s."),
+		   symtab_to_filename_for_display (cursal.symtab));
+	  print_source_lines (cursal.symtab,
+			      max (get_first_line_listed ()
+				   - get_lines_to_list (), 1),
+			      get_first_line_listed (), 0);
+	}
 
       return;
     }