This patch fixes a pair of TUI issues related to screen resizing:

1. In tui_handle_resize_during_io(), when the TUI screen gets resized,
we fail to update GDB's idea about the height of the output window.

You can see this bug by doing:

  a. Enter TUI mode.
  b. "show height"
  c. Resize the terminal.
  d. "show height"

And observe that despite resizing the terminal, the reported height
remains unchanged.  Note that a similar issue exists in the CLI.

The fix for this is simple: call tui_update_gdb_sizes() after performing
a resize, so that the "height" variable remains consistent with the
height of TUI's output window.

2. In tui_enable(), the call to tui_update_gdb_sizes() may clobber
readline's idea of the actual screen dimensions, and a subsequent
pending resize will use bogus terminal dimensions.

You can see this bug by doing:

  a. Enter TUI mode.
  b. Exit TUI mode.
  c. Resize the terminal.
  d. Enter TUI mode.
  e. Press a key to resize the screen.

And observe that the terminal gets incorrectly resized to the wrong
dimensions.  To fix this issue, we should oppurtunistically resize the
screen in tui_enable().  That way we eliminate the possibility of a
pending resize triggering right after we call tui_update_gdb_sizes().

gdb/ChangeLog:

	* tui/tui-io.c (tui_handle_resize_during_io): Call
	tui_update_gdb_sizes() after resizing the screen.
	* tui/tui.c (tui_enable): Resize the terminal before
	calling tui_update_gdb_sizes().