Add support for italic text on ttys.

Add support for italic text on ttys.

* src/dispextern.h: Replace unused TTY_CAP_BLINK with TTY_CAP_ITALIC.
(struct face): Remove unused fields tty_dim_p, tty_blinking_p, and
tty_alt_charset_p.  Add tty_italic_p.

* src/term.c: Support italics in capable terminals.
(no_color_bit): Replace unused NC_BLINK with NC_ITALIC.
(turn_on_face): Output using TS_enter_italic_mode if available.
Don't handle unused blinking and alt-charset cases.
(turn_off_face): Handle italic case; discard unused tty_blinking_p
and tty_alt_charset_p cases.
(tty_capable_p, init_tty): Support italics.

* src/termchar.h (struct tty_display_info): Add field for italics.
Remove unused blink field.

* src/xfaces.c (tty_supports_face_attributes_p, realize_tty_face):
Handle slant.