Use the CSS convention for #RGB colors (bug#36304)

Use the CSS convention for #RGB colors (bug#36304)

* src/xterm.c (x_parse_color): Change interpretation of #RGB color
triplets to match CSS rather than X conventions.

* lisp/term/tty-colors.el (tty-color-standard-values): Change
interpretation of #RGB color triplets to match CSS rather than X
conventions.  Allow upper-case digits.  Fix rgb:R/G/B
interpretation.

* doc/emacs/display.texi (Colors): Specify the convention used for
"#RGB" color triplets.

* test/lisp/tty-colors-tests.el: New file.

* etc/NEWS: Mention the change.