Make ANSI terminal escape sequences work in TUI

PR tui/14126 notes that ANSI terminal escape sequences don't affect
the colors shown in the TUI.  A simple way to see this is to try the
extended-prompt example from the gdb manual.

Curses does not pass escape sequences through to the terminal.
Instead, it replaces non-printable characters with a visible
representation, for example "^[" for the ESC character.

This patch fixes the problem by adding a simple ANSI terminal sequence
parser to gdb.  These sequences are decoded and those that are
recognized are turned into the appropriate curses calls.

The curses approach to color handling is unusual and so there are some
oddities in the implementation.

Standard curses has no notion of the default colors of the terminal.
So, if you set the foreground color, it is not possible to reset it --
you have to pick some other color.  ncurses provides an extension to
handle this, so this patch updates configure and uses it when
available.

Second, in curses, colors always come in pairs: you cannot set just
the foreground.  This patch handles this by tracking actually-used
pairs of colors and keeping a table of these for reuse.

Third, there are a limited number of such pairs available.  In this
patch, if you try to use too many color combinations, gdb will just
ignore some color changes.

Finally, in addition to limiting the number of color pairs, curses
also limits the number of colors.  This means that, when using
extended 8- or 24-bit color sequences, it may be possible to exhaust
the curses color table.

I am very sour on the curses design now.

I do not know how to write a test for this, so I did not.

gdb/ChangeLog
2018-12-28  Tom Tromey  <tom@tromey.com>

	PR tui/14126:
	* tui/tui.c (tui_enable): Call start_color and
	use_default_colors.
	* tui/tui-io.c (struct color_pair): New.
	(color_pair_map, last_color_pair, last_style): New globals.
	(tui_setup_io): Clean up color map when shutting down.
	(curses_colors): New constant.
	(get_color_pair, apply_ansi_escape): New functions.
	(tui_write): Rewrite.
	(tui_puts_internal): New function, from tui_puts.  Add "height"
	parameter.
	(tui_puts): Use tui_puts_internal.
	(tui_redisplay_readline): Use tui_puts_internal.
	(_initialize_tui_io): New function.
	(color_map): New globals.
	(get_color): New function.
	* configure.ac: Check for use_default_colors.
	* config.in, configure: Rebuild.

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