Support "replacement characters" on TTY frames

Support "replacement characters" on TTY frames

* src/nsterm.m (ns_draw_glyphless_glyph_string_foreground):
* src/pgtkterm.c (pgtk_draw_glyphless_glyph_string_foreground):
* src/haikuterm.c (haiku_draw_glyphless_glyph_string_foreground):
* src/xterm.c (x_draw_glyphless_glyph_string_foreground):
* src/w32term.c (w32_draw_glyphless_glyph_string_foreground):
* src/xdisp.c (lookup_glyphless_char_display): Handle extra-slot
of 'glyphless-char-display' that is a cons cell.
(syms_of_xdisp) <glyphless-char-display>: Update doc string.

* etc/NEWS:
* doc/lispref/display.texi (Glyphless Chars): Document the new
feature.

* lisp/faces.el (glyphless-char): Make the face stand out on TTY
frames that don't support the underline attribute.