Use ISCNTRL in tui_copy_source_line

Use ISCNTRL in tui_copy_source_line

This changes tui_copy_source_line to use ISCNTRL.  This lets it work
more nicely with UTF-8 input.  Note that this still won't work for
stateful multi-byte encodings; for that much more work would be
required.  However, I think this patch does not make gdb any worse in
this scenario.

gdb/ChangeLog
2020-09-27  Tom Tromey  <tom@tromey.com>

	PR tui/25342:
	* tui/tui-winsource.c (tui_copy_source_line): Use ISNCTRL.