Fix PR tui/21216: TUI line breaks regression

Fix PR tui/21216: TUI line breaks regression

Commit d7e747318f4d04 ("Eliminate make_cleanup_ui_file_delete / make
ui_file a class hierarchy") regressed the TUI's command window.
Newlines miss doing a "carriage return", resulting in output like:

~~~~~~~~~~~~~~~~~~
(gdb) helpList of classes of commands:

                                      aliases -- Aliases of other commands
                                                                          breakpoints -- Making program stop at certain points
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Before the commit mentioned above, the default ui_file->to_write
implementation had a hack that would defer into the ui_file->to_fputs
method.  The TUI's ui_file did not implement the to_write method, so
all writes would end up going to the ncurses window via tui_file_fputs
-> tui_puts.

After the commit above, the hack is gone, but the TUI's ui_file still
does not implement the ui_file::write method.  Since tui_file inherits
from stdio_file, writing to a tui_file ends up doing fwrite on the
FILE stream the TUI is "associated" with, via stdio_file::write,
instead of writing to the ncurses window.

The fix is to have tui_file override the "write" method.

New test included.

gdb/ChangeLog:
2017-03-08  Pedro Alves  <palves@redhat.com>

	PR tui/21216
	* tui/tui-file.c (tui_file::write): New.
	* tui/tui-file.h (tui_file): Override "write".
	* tui/tui-io.c (do_tui_putc, update_start_line): New functions,
	factored out from ...
	(tui_puts): ... here.
	(tui_putc): Use them.
	(tui_write): New function.
	* tui/tui-io.h (tui_write): Declare.

gdb/testsuite/ChangeLog:
2017-03-08  Pedro Alves  <palves@redhat.com>

	PR tui/21216
	* gdb.tui/tui-nl-filtered-output.exp: New file.