patch 9.1.1054: Vim doesn't work well with TERM=xterm-direct

patch 9.1.1054: Vim doesn't work well with TERM=xterm-direct

Problem:  Vim doesn't work well with TERM=xterm-direct
          (Andrea Pappacoda)
Solution: detect if a terminal supports true-colors and
          enable termguicolors

The terminfo database for xterm-direct contains both the (non-standard)
termcap RGB capability and a number of colors == 0x1000000 so it seems
either of those two options can be used to detect a terminal capable of
displaying true colors.

So set the termguicolor option automatically, when either of the two
options is detected. (for some reasons, my debian xterm (v393) does not
respond to XTGETTCAP query attempts, so falling back to the number of
colors seems like a good compromize)

fixes: #16327
closes: #16490

Signed-off-by: Christian Brabandt <cb@256bit.org>